Production of branched-chain alcohols by photosynthetic microorganisms

ABSTRACT

The present invention provides genes, polypeptides and expression constructs therefor, recombinant photosynthetic microorganisms, and method of use thereof, such as for the production of branched-chain alcohols (including 2-methyl-1-butanol, 3-methyl-1-butanol, and isobutanol) and derivatives thereof for a variety of uses.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisional Application No. 61/121,522, filed Dec. 10, 2008, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention provides genes, polypeptides, and expression constructs therefor, recombinant photosynthetic microorganisms and methods of uses therefor, such as the production of branched-chain alcohols, which can optionally be derivatized to produce further compounds. This biological route provides advantages over other known methods of production.

INCORPORATION BY REFERENCE OF SEQUENCE LISTING

This application contains references to amino acid sequences and/or nucleic acid sequences which have been submitted concurrently herewith as the sequence listing text file “616872004000SeqList.txt”, file size 25.0 KiloBytes (KB), created on Dec. 10, 2009. The aforementioned sequence listing is hereby incorporated by reference in its entirety pursuant to 37 C.F.R. §1.52(e)(5).

BACKGROUND

Branched-chain alcohols and their derivatives have a variety of utilities that are known in the art, for example, as fuels, fuel additives, and solvents. Advantages of using branched-chain alcohols such as isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol as fuels or fuel additives include an energy content higher than that of ethanol and their ability to be transported using existing fuel pipelines. Traditional methods for the production of branched-chain alcohols are costly, for example fermentation requires a fermentable carbon source, typically a sugar or polysaccharide, which adds to the cost of production.

US 2007/0092957 discloses synthesis of isobutanol by recombinant nonphotosynthetic bacteria and yeast that utilize glucose or sucrose as carbon sources. US 2007/0259411 describes the selection of butanol-tolerant bacterial Enterococcus species in a growth medium that includes a fermentable carbon source for the production of alcohols. US 2008/0261230 provides genes encoding high activity keto-acid reductoisomerases that can be used to genetically engineer microorganisms for the production of isobutanol. US 2009/0081746 discloses the synthesis of branched-chain alcohols including isobutanol, 1-butanol, 1-propanol, 2-methyl 1-butanol, 3-methyl-1-butanol, and 2-phenylethanol by recombinant E. coli strains in cultures that include glucose.

US 2009/0288337 (application Ser. No. 12/332,305) and WO 2009/076480 (PCT application US2008/086296), both incorporated by reference herein, describe genetically engineered microorganisms such as bacteria and yeast for the synthesis of 2-methyl-1-butanol.

SUMMARY OF THE INVENTION

This invention provides polypeptides, genes, expression constructs, metabolic pathways, strains of photosynthetic microorganisms, and methods to biologically produce branched-chain alcohols, including, for example, 2-methyl-1-butanol, 3-methyl-1-butanol, and isobutanol.

One aspect of this invention involves the production of recombinant photosynthetic microorganisms via introduction of heterologous genes that encode enzymes that enhance the production and decarboxylation of 2-keto branched-chain acids, leading to the production of the corresponding branched-chain aldehydes. Additional gene introductions can then be carried out to enable the efficient reduction of the branched-chain aldehydes to the corresponding branched-chain alcohols. In addition, the invention provides methods where branched-chain alcohols are enzymatically dehydrated in vivo to produce various branched-chain alpha-olefins.

In one embodiment, the invention provides a recombinant photosynthetic microorganism that includes at least one heterologous DNA sequence encoding at least one polypeptide that catalyzes a substrate to product conversion that leads to the synthesis of isobutanol. The encoded polypeptide can be a polypeptide that catalyzes the conversion of: (1) pyruvate to 2-hydroxy-2-methyl-3-oxobutanoate; (2) 2-hydroxy-2-methyl-3-oxobutanoate to 2,3-dihydroxy-3-methylbutanoate; (3) 2,3-dihydroxy-3-methylbutanoate to 3-methyl-2-oxobutanoate; (4) 3-methyl-2-oxobutanoate to 2-methyl-1-propanal; or (5) 2-methyl-1-propanal to 2-methyl-1-propanol (isobutanol).

In another embodiment, the invention provides a recombinant photosynthetic microorganism that includes at least one heterologous DNA sequence encoding at least one polypeptide that catalyzes a substrate to product conversion selected that leads to the synthesis of 2-methyl-1-butanol. The encoded polypeptide can be a polypeptide that catalyzes the conversion of: (6) pyruvate to 2-methylmalate; (7) 2-methylmalate to 2-methylmaleate; (8) 2-methylmaleate to D-erythro-3-methylmalate; (9) D-erythro-3-methylmalate to 2-oxobutanoate; (10) threonine to 2-oxobutanoate; (11) pyruvate and 2-oxobutanoate to 2-hydroxy-2-ethyl-3-oxobutanoate; (12) 2-hydroxy-2-ethyl-3-oxobutanoate to 2,3-dihydroxy-3-methylpentanoate; (13) 2,3-dihydroxy-3-methylpentanoate to 3-methyl-2-oxopentanoate; (14) 3-methyl-2-oxopentanoate to 2-methyl-1-butanal; and (15) 2-methyl-1-butanal to 2-methyl-1-butanol.

In another embodiment, the invention provides a recombinant photosynthetic microorganism that includes at least one heterologous DNA sequence encoding at least one polypeptide that catalyzes a substrate to product conversion that leads to the synthesis of 3-methyl-1-butanol. The encoded polypeptide can be a polypeptide that catalyzes the conversion of: (16) pyruvate to 2-hydroxy-2-methyl-3-oxobutanoate; (17) 2-hydroxy-2-methyl-3-oxobutanoate to 2,3-dihydroxy-3-methylbutanoate; (18) 2,3-dihydroxy-3-methylbutanoate to 3-methyl-2-oxobutanoate; (19) 3-methyl-2-oxobutanoate to 2-isopropylmalate; (20) 2-isopropylmalate to 2-isopropylmaleate; (21) 2-isopropylmaleate to 3-isopropylmalate; (22) 3-isopropylmalate to 4-methyl-2-oxopentanoate (2-ketoisocaproate); (23) 4-methyl-2-oxopentanoate to 3-methyl-1-butanal; and (24) 3-methyl-1-butanal to 3-methyl-1-butanol.

In some embodiments, provided herein are recombinant photosynthetic microorganisms that include a heterologous nucleic acid molecule that encodes a branched-chain 2-ketoacid decarboxylase, in which the photosynthetic microorganism produces a branched-chain alcohol such as isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol. In an exemplary embodiment, a photosynthetic microorganism comprises a heterologous nucleic acid sequence encoding kdcA of Lactococcus lactis or a variant thereof. In another exemplary embodiment, a photosynthetic microorganism comprises a heterologous nucleic acid sequence encoding PDC3-6 of Pichia stipilis or a variant thereof.

In some embodiments, recombinant photosynthetic microorganisms are provided that produce one or more branched-chain alcohols, in which the photosynthetic microorganisms include a heterologous nucleic acid sequence encoding an alcohol dehydrogenase, such as a branched-chain alcohol dehydrogenase. In an exemplary embodiment, a recombinant photosynthetic microorganism includes a heterologous nucleic acid sequence encoding ADH6 of Saccharomyces cerevisiae, or a variant thereof.

In further embodiments, a recombinant photosynthetic microorganism is genetically engineered for the production of one or more branched-chain alcohols, such as isobutanol, 2-methyl-1-butanol, or 3-methyl-1 butanol, includes at least one heterologous nucleic acid sequence encoding one or more enzymes selected from the group consisting of an acetolactate synthase (EC 2.2.1.6), a ketol-acid reductoisomerase (EC 1.1.1.86), or dihydroxyacid dehydratase (EC 4.2.1.9). In some exemplary embodiments, a recombinant photosynthetic microorganism includes a heterologous nucleic acid sequence encoding an acetolactate synthase (EC 2.2.1.6).

Recombinant photosynthetic microorganisms in certain embodiments of the invention are engineered to produce 3-methyl-1-butanol and include one or more heterologous nucleic acid sequences encoding one or more of the enzymes 2-isopropylmalate synthase (EC 2.3.3.13), 3-isopropylmalate dehydratase (EC 4.2.1.33), or 3-isopropylmalate dehydrogenase (EC 1.1.1.85).

In other embodiments, a recombinant photosynthetic microorganism is genetically engineered to produce 2-methyl-1-butanol, and includes at least one heterologous nucleic acid sequence encoding one or more of the enzymes homoserine dehydrogenase (EC 1.1.1.3), homoserine kinase (EC 2.7.1.39), threonine synthase (EC 4.2.3.1), or threonine ammonia-lyase (EC 4.3.1.19).

A further aspect of the invention is a method for producing a branched-chain alcohol in which the method includes culturing a recombinant photosynthetic microorganism as provided herein, such as a microorganism that includes a heterologous sequence encoding a branched-chain 2-ketoacid decarboxylase and a heterologous sequence encoding an alcohol dehydrogenase, to produce a branched-chain alcohol. In some preferred embodiments the photosynthetic microorganism is cultured photoautotrophically. In some embodiments, the photosynthetic microorganism is cultured in the absence of a reduced carbon source, such as a sugar or organic acid. The photosynthetic microorganism culture is in some embodiments provided with inorganic carbon such as CO₂, carbonic acid, or a carbonate salt.

A photosynthetic microorganism used in the methods may further include at least one heterologous nucleic acid sequence encoding one or more of an acetolactate synthase (EC 2.2.1.6), a ketol-acid reductoisomerase (EC 1.1.1.86), or dihydroxyacid dehydratase (EC 4.2.1.9). A branched-chain alcohol produced by the culture is in some preferred embodiments isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol.

In some embodiments, the photosynthetic microorganism produces 3-methyl-1-butanol. In some embodiments, the photosynthetic microorganism produces 3-methyl-1-butanol and is engineered to include at least one heterologous nucleic acid sequence encoding one or more of the enzymes 2-isopropylmalate synthase (EC 2.3.3.13), 3-isopropylmalate dehydratase (EC 4.2.1.33), or 3-isopropylmalate dehydrogenase (EC 1.1.1.85).

In some embodiments, the photosynthetic microorganism produces 2-methyl-1-butanol. In some embodiments, the photosynthetic microorganism produces 2-methyl-1-butanol and is engineered to include at least one nucleic acid sequence encoding one or more of the enzymes homoserine dehydrogenase (EC 1.1.1.3), homoserine kinase (EC 2.7.1.39), threonine synthase (EC 4.2.3.1), or threonine ammonia-lyase (EC 4.3.1.19).

In some embodiments, the method includes recovering the branched-chain alcohol from the culture medium, for example, using methods such as distillation, liquid-liquid extraction, gas stripping, steam stripping, and/or pervaporation.

In another aspect, included within the scope of the invention is a branched-chain alcohol made by the methods provided herein. The branched-chain alcohol produced by a recombinant photosynthetic microorganism can be, for example, isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a biochemical pathway for the synthesis of isobutanol that overlaps with the biosynthetic pathway for the amino acid valine and a biochemical pathway for the synthesis of 3-methyl-1-butanol that overlaps with the biosynthetic pathway for the amino acid leucine.

FIG. 2 depicts a biochemical pathway 2-methyl-1-butanol that overlaps with the biosynthetic pathway for the amino acid isoleucine.

FIG. 3 provides SEQ ID NO:1, the sequence of an operon for expression in Synechococcus elongatus including codon-optimized Saccharomyces cerevisiae pyruvate decarboxylase gene PDC1, the S. elongatus KaiBC intergenic region, the codon-optimized S. cerevisiae alcohol dehydrogenase gene ADH2, and the rrnB terminator.

FIG. 4 provides SEQ ID NO:2, the sequence of an operon for expression in Synechococcus elongatus including a 2-ketoacid decarboxylase gene from Lactococcus lactis (KDCa) in combination with a codon-optimized S. cerevisiae ADH2 gene.

FIG. 5 provides SEQ ID NO:3, the sequence of an operon including the S. cerevisiae ADH6 gene in combination with the codon-optimized S. cerevisiae PDC1 gene.

FIG. 6 provides SEQ ID NO:4, the sequence of an operon including the L. lactis KDCa gene in combination with the S. cerevisiae ADH6 gene.

FIG. 7 provides SEQ ID NO:7, the sequence of an expression construct that includes the trc promoter, and the Synechocystis sp. PCC 6803 ilvB coding sequence and the rps14 terminator.

FIG. 8 provides SEQ ID NO:8, the sequence of the branched-chain 2-ketoacid decarboxylase protein encoded by the PDC3-6 gene of Pichia stipitis.

DETAILED DESCRIPTION OF THE INVENTION

This application incorporates by reference U.S. patent application Ser. No. 12/332,305; PCT application PCT/US08/086296; and U.S. provisional application 60/012,749 in their entireties.

All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention is related. The following terms are defined for purposes of the invention as described herein.

The singular form “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes a plurality of cells and reference to “an antibody” includes a plurality of antibodies, etc.

As used herein, the terms “about” or “approximately” when referring to any numerical value are intended to mean a value of plus or minus 10% of the stated value. For example, “about 50 degrees C.” (or “approximately 50 degrees C.”) encompasses a range of temperatures from 45 degrees C. to 55 degrees C., inclusive. Similarly, “about 100 mM” (or “approximately 100 mM”) encompasses a range of concentrations from 90 mM to 110 mM, inclusive. All ranges provided within the application are inclusive of the values of the upper and lower ends of the range.

An “isolated” biomolecule such as an isolated protein or nucleic acid, is a biomolecule removed from the context in which the biomolecule exist in nature. For example, an isolated protein or nucleic acid molecule is removed from the cell or organism with which it is associated in its natural state. An isolated biomolecule can be, in some instances, partially or substantially purified, for example, an isolated nucleic acid molecule can be a nucleic acid sequence that has been excised from the chromosome, genome, or episome that it is integrated into in nature.

A recombinant or “engineered” nucleic acid molecule is a nucleic acid molecule that has been altered through human intervention. As nonlimiting examples, a recombinant nucleic acid molecule: 1) includes conjoined nucleotide sequences that are not conjoined in nature, 2) has been engineered using molecular cloning techniques such that it lacks one or more nucleotides with respect to the naturally occurring nucleic acid molecule sequence, or 3) has been manipulated using molecular cloning techniques such that it has one or more sequence changes or rearrangements with respect to the naturally occurring nucleic acid sequence. As nonlimiting examples, a cDNA is a recombinant DNA molecule, as is any nucleic acid molecule that has been generated by in vitro polymerase reaction(s), or to which linkers have been attached, or that has been integrated into a vector, such as a cloning vector or expression vector.

A recombinant or “engineered” organism is an organism into which one or more recombinant or “engineered” nucleic acid molecules has been introduced.

A “homolog” of a gene or protein refers to its functional equivalent in another species.

A “variant” of a gene or nucleic acid sequence is a sequence having at least 65% identity with the referenced gene or nucleic acid sequence, and can include one or more base deletions, additions, or substitutions with respect to the referenced sequence. Variants also include chimeric genes that include sequences from two or more sources. Variants also include codon-optimized genes, and genes containing mutations, insertions, deletions, or substitutions, either naturally-occurring or recombinant. A variant can be a naturally-occurring variant or the result of a spontaneous or induced mutation. Induced mutations can be created using methods known in the art for mutagenesis of organisms or cells (for example, using gamma or UV irradiation or chemical mutagens such as 5-bromo deoxyuridine, ethyl methane sulfonate (EMS), methyl methane sulfonate (MMS), diethylsulfate (DES), nitrosoguanidine (NTG), ICR compounds, etc., or can be introduced using genetic engineering techniques, such as gene synthesis, in vivo single strand repair techniques, polymerase-based amplification at error-permissive temperature and/or polymerase-based amplification using primers that incorporate base changes.

A “variant” of a peptide or protein is a peptide or protein sequence that varies at one or more amino acid positions with respect to the reference peptide or protein. A variant can be a naturally-occurring variant or can be the result of spontaneous, induced, or genetically engineered mutation(s) to the nucleic acid molecule encoding the variant peptide or protein. A variant peptide can also be a chemically synthesized variant.

The degree of amino acid or nucleic acid sequence identity can be determined by various computer programs for aligning the sequences to be compared based on designated program parameters. For example, sequences can be aligned and compared using the local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), the homology alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), or the search for similarity method of Pearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), and can be aligned and compared based on visual inspection or can use computer programs for the analysis (for example, GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.).

The BLAST algorithm, described in Altschul, et al., J. Mol. Biol. 215:403-410 (1990), is publicly available through software provided by the National Center for Biotechnology Information (at the World Wide Web address ncbi.nlm.nih.gov). This algorithm identifies high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul, et al., supra.). Initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always >0) and N (penalty score for mismatching residues; always <0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. For determining the percent identity of an amino acid sequence or nucleic acid sequence, the default parameters of the BLAST programs can be used. For analysis of amino acid sequences, the BLASTP defaults are: word length (W), 3; expectation (E), 10; and the BLOSUM62 scoring matrix. For analysis of nucleic acid sequences, the BLASTN program defaults are word length (W), 11; expectation (E), 10; M=5; N=−4; and a comparison of both strands. The TBLASTN program (using a protein sequence to query nucleotide sequence databases) uses as defaults a word length (W) of 3, an expectation (E) of 10, and a BLOSUM 62 scoring matrix. (see Henikoff & Henikoff, Proc. Natl. Acad. Sci. USA 89:10915 (1989)).

In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Nat'l. Acad. Sci. USA 90:5873-5787 (1993)). The smallest sum probability (P(N)), provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.1, preferably less than about 0.01, and more preferably less than about 0.001.

“Exogenous” in the context of a gene or protein is a gene or protein that is not derived from the host organism species.

A “heterologous” gene or nucleic acid sequence is a gene or sequence from a different source than the host organism it is introduced into, or from a different source than another nucleic acid sequence with which is juxtaposed in a nucleic acid construct. For example, a gene of one species introduced into another species may be referred to as a heterologous gene. A nucleic acid molecule that includes a gene operably linked to a promoter that is not the natural promoter for the gene (not the promoter linked to the gene in its natural state) is also referred to herein as a heterologous nucleic acid molecule or sequence, even though the gene may be derived from the same species as the host organism.

A gene that is “codon-optimized” for expression in an organism is a gene whose nucleotide sequence has been altered with respect to the original nucleotide sequence, such that one or more codons of the nucleotide sequence has been changed to a different codon that encodes the same amino acid, in which the new codon is used more frequently in genes of the organism of interest than the original codon. The degeneracy of the genetic code provides that all amino acids except form methionine and tryptophan are encoded by more than one codon. For example, arginine, leucine, and serine are encoded by different six different codons; glycine, alanine, valine, threonine, and proline are encoded by four different codons. Many organisms use certain codons to encode a particular amino acid more frequently than others. Without limiting any aspects of the invention to any particular mechanism, it is believed that some tRNAs for a given amino acid are more prevalent than others within a particular organism, and genes requiring a rare tRNA for translation of the encoded protein may be expressed at a low level due in part to a limiting amount of the rare tRNA. Thus, for adequate or optimal levels of expression of an encoded protein, a gene may be “codon-optimized” to change one or more codons to new codons (“preferred codons”) that are among those used more frequently in the genes of the host organism (referred to as the “codon preference” of the organism). As used in the context of the invention, a “codon-optimized” gene or nucleic acid molecule of the invention need not have every codon altered to conform to the codon preference of the intended host organism, nor is it required that altered codons of a “codon-optimized” gene or nucleic acid molecule be changed to the most prevalent codon used by the organism of interest. For example, a codon-optimized gene may have one or more codons changed to codons that are used more frequently that the original codon(s), whether or not they are used most frequently in the organism to encode a particular amino acid.

A “photosynthetic microorganism” is any prokaryotic or eukaryotic single-celled or colonial organism that can perform photosynthesis and that can be seen as a single organism only with the aid of a microscope. Photosynthetic microorganisms include eukaryotic microalgae and photosynthetic bacteria. Eukaryotic microalgae include species of green algae (Chlorophyceae), yellow-green algae (Xanthophyceae), golden algae (Chrysophyceae), brown algae (Phaeophyceae), red algae (Rhodophyceae), diatoms (Bacillariophyceae), and “pico-plankton” (Prasinophyceae and Eustigmatophyceae). Photosynthetic bacteria include cyanobacteria, green sulfur bacteria, purple sulfur bacteria, purple nonsulfur bacteria, and green nonsulfur bacteria.

Photoautotrophic growth or culture means the growth of organisms in the absence of a supplied compound or molecule that can be metabolized for energy (such as a reduced carbon source) and under conditions in which the organisms use light as the sole energy source.

Inorganic carbon is carbon provided in a molecule that cannot itself be metabolized for energy by an organism, such as CO₂, carbonic acid, and carbonate. Sources of inorganic carbon include CO₂, air, carbonic acid, carbonate salts, and emissions such as flue gas.

Carbon dioxide (which, along with carbonic acid, bicarbonate and/or carbonate define the term “inorganic carbon”) is converted in the photosynthetic process to organic compounds. The inorganic carbon source includes any way of delivering inorganic carbon, optionally in admixture with any other combination of compounds which do not serve as the primary carbon feedstock, but only as a mixture or carrier (for example, emissions from biofuel (e.g., ethanol) plants, power plants, petroleum-based refineries, as well as atmospheric and subterranean sources).

A reduced or organic carbon source is a carbon based molecule that can be metabolized by an organism for energy such as, for example, a carbohydrate (including a sugar or polysaccharide), amino acid, protein, organic acid, fatty acid, lipid, acetyl CoA, or any biosynthetic precursor of any of these biomolecules.

Elements of the embodiments described herein can be combined to make additional embodiments not specifically described that are also within the scope of the invention. Headings within the application are solely for the convenience of the reader, and do not limit in any way the scope of the invention or its embodiments.

In one aspect, the invention includes engineering a recombinant photosynthetic microorganism to produce various branched-chain alcohol molecules. In preferred embodiments, the branched-chain alcohols the photosynthetic microorganisms are engineered to produce are five carbon branched-chain alcohols that can be synthesized using, in part, enzymes that catalyze substrate to product conversions on certain amino acid biosynthesis pathways.

One embodiment of this invention is to express in a photosynthetic microorganism one or more heterologous genes that encode enzymes involved in the production of branched-chain alcohols including isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol. The synthesis of each of these products utilizes enzymes of an endogenous amino acid biosynthesis pathway. The recombinant microorganisms are engineered to include one or more heterologous genes encoding enzymes which, in combination with endogenous enzymes, result in synthesis of the branched-chain alcohols. Amino acid biosynthesis pathways for producing isoleucine, valine, and leucine, as well as additional enzymes for the production of isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol that are not part of the endogenous amino acid pathways, are provided in FIGS. 1 and 2.

In one embodiment, the present invention provides methods of producing isobutanol. Each step of the enzymatic pathway is provided with a numeric designation which corresponds to an polypeptide with enzymatic activity to perform the following substrate to product conversions (FIG. 1):

-   (1) pyruvate to 2-hydroxy-2-methyl-3-oxobutanoate; -   (2) 2-hydroxy-2-methyl-3-oxobutanoate to     2,3-dihydroxy-3-methylbutanoate; -   (3) 2,3-dihydroxy-3-methylbutanoate to 3-methyl-2-oxobutanoate; -   (4) 3-methyl-2-oxobutanoate to 2-methyl-1-propanal; and -   (5) 2-methyl-1-prop anal to 2-methyl-1-prop anol (or isobutanol).

In another embodiment, the present invention provides methods of producing 2-methyl-1-butanol. Each step of the enzymatic pathway is provided with a numeric designation which corresponds to an polypeptide with enzymatic activity to perform the following substrate to product conversions (FIG. 2):

-   (6) pyruvate to 2-methylmalate; -   (7) 2-methylmalate to 2-methylmaleate; -   (8) 2-methylmaleate to D-erythro-3-methylmalate; -   (9) D-erythro-3-methylmalate to 2-oxobutanoate; -   (10) threonine to 2-oxobutanoate; -   (11) Pyruvate and 2-oxobutanoate to     2-hydroxy-2-ethyl-3-oxobutanoate; -   (12) 2-hydroxy-2-ethyl-3-oxobutanoate to     2,3-dihydroxy-3-methylpentanoate; -   (13) 2,3-dihydroxy-3-methylpentanoate to 3-methyl-2-oxopentanoate; -   (14) 3-methyl-2-oxopentanoate to 2-methyl-1-butanal; and -   (15) 2-methyl-1-butanal to 2-methyl-1-butanol.

In another embodiment, the present invention provides methods of producing 3-methyl-1-butanol. Each step of the enzymatic pathway is provided with a numeric designation which corresponds to an polypeptide with enzymatic activity to perform the following substrate to product conversions (FIG. 1):

-   (16) pyruvate to 2-hydroxy-2-methyl-3-oxobutanoate; -   (17) 2-hydroxy-2-methyl-3-oxobutanoate to     2,3-dihydroxy-3-methylbutanoate; -   (18) 2,3-dihydroxy-3-methylbutanoate to 3-methyl-2-oxobutanoate; -   (19) 3-methyl-2-oxobutanoate to 2-isopropylmalate; -   (20) 2-isopropylmalate to 2-isopropylmaleate; -   (21) 2-isopropylmaleate to 3-isopropylmalate; -   (22) 3-isopropylmalate to 4-methyl-2-oxopentanoate     (2-ketoisocaproate); -   (23) 4-methyl-2-oxopentanoate to 3-methyl-1-butanal; and -   (24) 3-methyl-1-butanal to 3-methyl-1-butanol.

Gene variants encoding enzymes having 80-85% identity, 85%-90% identity, 90%-95% identity, or 95%-100% identity with amino acid sequences of the enzymes disclosed herein, in which the encoded enzymes have at least the activity of the reference enzymes, are contemplated for use in the recombinant photosynthetic microorganisms of the invention. Such gene variants may be tested for use in engineering host strains of the invention that produce one or more branched-chain alcohols using methods provided in this application or in US patent application publication 2009/0288337, incorporated by reference.

In some aspects, the invention provides a recombinant photosynthetic microorganism that includes a heterologous nucleic acid sequence that encodes a 2-ketoacid decarboxylase, in which the photosynthetic microorganism produces a branched-chain alcohol. Exemplary polypeptides catalyzing the substrate to product conversions of reactions (4), (14), and (23) include 2-oxo-acid carboxy-lyase enzymes (also referred to as alpha-ketoacid decarboxylase enzymes), including enzymes in the EC 4.1.1.1, EC 4.1.1.43, EC 4.1.1.71, EC 4.1.1.72, EC 4.1.1.74 classes, that are able to utilize branched-chain 2-oxo-carboxylic acids as substrates to produce branched-chain aldehydes. A number of genes have been identified that encode 2-oxo-acid carboxy-lyase enzymes that can be tested for activity on branched-chain 2-oxo-carboxylic acids (e.g., 2-oxo-3-methylbutanoate, 3-methyl-2-oxopentanoate, and 4-methyl-2-oxopentanoate) and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): P83779; Q6FJA3; Q12629; P33149; P28516; A2Y5L9; Q0DHF6; P51850; Q09737; P51845; P06169; Q05326; A2XFI3; Q10MW3; P51851; Q92345; P51846; Q05327; A2YQ76; Q0D3D2; Q9P7P6; O42873; P16467; P26263; Q4WXX9; Q96535; Q684K0; Q84V95; Q5BN14; Q5BN15; Q7M227; Q96536; B0ZS79; Q9SM49; B3F7U5; Q7M228; B2J634; Q1QC58; Q3EJQ4; Q8L388; Q93EN4; Q9R5L0; Q8KTX6; B0VBZ7; B0VUA9; B3Q3J2; Q5ZWD0; Q2JYJ7; Q2YVJ9; Q5FRZ6; Q4FTE7; Q2YUZ2; Q93IM7; Q2UKV4; P51844; Q0CNV1; P87208; P34734; P33287; P06672; Q0W4D3; A2Q7Q7; A3GF21; A3GGL8; O43107; Q659I2; Q8NK65; Q9UUT6; Q8NK64; A2QT68; Q4W928; A2R228; A4HQP2; A5AA75; B0DZR5; Q4WW88; O43106; Q43005; Q7U0A6; A1KI36; Q9CC97; Q73WX4; A0R2B1; A3Q3N5; A1UK81; Q1B4V6; A5U1U6; O50463; A0PVU7; A1TDK2; Q06408; A3LXV3; Q07471; Q6QBS4; Q684J7; Q9CG07; O53865; and A4F1Y5. In some embodiments, a 2-ketoacid decarboxylase enzyme encoded by a nucleic acid molecule used to engineer a photosynthetic host organism is encoded by PDC1 (protein accession number CAA97573; UniProtKB P06169) PDC5 (CAA97705; UniProtKB P16467), PDC6 (CAA39398; UniProtKB P26263), THI3 (CAA98646; UniProtKB Q07471), or ARO10 (AAB64816; UniProtKB Q06408), of Saccharomyces cereviseae; PDC1 (UniProtKB A3GGL8), PDC2 (UniProtKB A3GF21), PDC3 (ABN67867.1 GI:126093174; UniProtKB A3LXV3), or PDC6 (also known as PDC3-6; ABN67867.1 GI:126093174; SEQ ID NO:8) of Pichia stipitis; kdcA (AAS49166; GI:44921617; UniProtKB Q6QBS4), kdcA-S286Y, kdcA-F381W, or kdcA-S286Y, F381W of Lactococcus lactis; Kdc of Mycobacterium tuberculosis (UniProtKB 053865); or indolepyruvate decarboxylase of Salmonella enterica (NP_(—)461346; GI:16765731), or any genes encoding a 2-ketoacid decarboxylase or a pyruvate decarboxylase disclosed in US 2009/0288337, incorporated by reference herein. Any of these genes, variants or homologs of these genes, or others known to be or suspected of being genes that encode branched-chain 2-ketoacid decarboxylases, may be tested for use in engineering host strains of the invention that produce one or more branched-chain alcohols.

In some aspects, the invention provides a recombinant photosynthetic microorganism that includes a heterologous nucleic acid sequence that encodes an alcohol dehydrogenase, in which the photosynthetic microorganism produces a branched-chain alcohol, such as isobutanol, 3-methyl-1-butanol, or 2-methyl-1-butanol. Exemplary polypeptides catalyzing the substrate to product conversions of reactions (5), (15), and (24) include alcohol dehydrogenase enzymes (EC 1.1.1.1 and EC 1.1.1.2) that are able to utilize branched-chain aldehydes as substrates to produce the corresponding branched-chain alcohols. A number of genes have been identified as alcohol dehydrogenases that can be tested for activity on branched-chain aldehydes and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): P07327; P28469; Q5RBP7; P25405; P00325; Q5R1W2; P14139; P25406; P00327; P00326; O97959; P00328; P80222; P30350; P49645; P06525; P41747; P12311; Q17334; P43067; P85440; P48814; Q70UN9; P23991; P19631; P23236; P48586; P09370; P22246; P07161; P12854; P08843; P26325; Q9Z2M2; Q64413; Q64415; P05336; P20369; Q07288; P00333; P00329; P80512; Q9P6C8; Q75ZX4; Q2R8Z5; P12886; P14219; P41680; P25141; O00097; Q03505; P22797; P06757; P14673; P80338; P13603; P00330; Q07264; P20368; P42327; O45687; O94038; P48815; Q70UP5; Q70UP6; P27581; P25720; P23237; P48587; P09369; P07160; P24267; P37686; P54202; Q24803; P10847; P49383; Q9P4C2; P04707; Q4R1E8; Q0ITW7; O13309; P28032; P14674; P00331; P06758; P42328; P07754; P10848; P49384; P14675; P07246; P08319; P49385; Q9QYY9; Q64563; Q09669; P80468; A6ZTT5; P10127; Q6XQ67; P38113; P28332; P41681; Q5R7Z8; Q5XI95; P40394; Q64437; P41682; O31186; Q7U1B9; P71818; P33744; P0A9Q8; P0A9Q7; P81600; P72324; Q9SK86; Q9SK87; A1L4Y2; Q8VZ49; Q0V7W6; Q8LEB2; Q9FH04; P81601; P39451; O46649; O46650; Q96533; Q3ZC42; Q17335; Q54TC2; P46415; P19854; P11766; P93629; P28474; P80360; P81431; A2XAZ3; Q0DWH1; P80572; O19053; P12711; P79896; P80467; Q9NAR7; Q00669; P21518; P25139; P48584; Q00670; P22245; Q9NG42; P28483; P48585; P51551; Q09009; P51549; P21898; Q07588; Q9NG40; Q27404; P10807; P07162; Q09010; P00334; Q00671; P25721; Q00672; P07159; P84328; P37473; P23361; P23277; Q6LCE4; Q9U8S9; Q9GN94; Q24641; P23278; Q03384; P28484; P51550; Q05114; P26719; P17648; P48977; P81786; P14940; P25988; P00332; Q2FJ31; Q2G0G1; Q2YSX0; Q5HI63; Q99W07; Q7A742; Q6GJ63; Q6GBM4; Q8NXU1; Q5HRD6; Q8CQ56; Q4J781; P39462; P50381; Q96XE0; P51552; P32771; A7ZIA4; Q8X5J4; A7ZX04; A1A835; Q0TKS7; Q8FKG1; B1J085; P25437; B1LIP1; Q1RFI7; P44557; P39450; Q3Z550; P73138; P71017; P33010; P35630; Q24857; Q04894; P25377; O57380; P0A4X1; P0A4X0; P25984; P75214; P14941; Q3ZCJ2; O70473; P14550; Q9JII6; P50578; P51635; Q9UUN9; and P27800. In some embodiments an alcohol dehydrogenases that may be encoded by a nucleic acid molecule used to engineer a photosynthetic organism for the production of a branched-chain alcohol is encodes by ADH1 (Genbank protein accession number CAA58193), ADH2 (AAA34408;), ADH3 (CAA89229), ADH6 (CAA90836; GI:984691), ADH7 (CAA4223), GRE2 (CAA88277), SFA1 (CAA91578), or YPR1 (CAA56686) of Saccharomyces cereviseae; ADH3 (ABN65575), ADH6 (EAZ62840), ADH7 (CAA42237), or GRE2 (CAA88277), of Pichia stipitis; ADH1 (ABK75278), ADHs (AAK45115), or adhb (CAE55322) of Mycoplasma tuberculosis; yqhD of E. coli (Genbank accession YP_(—)001745276, GI:170682079), or ADHE of Equus caballus (P00327), or any genes encoding alcohol dehydrogenases disclosed in US 2009/0288337, incorporated by reference herein. Any of these genes, variants or homologs of these genes, or others known to be or suspected of being genes that encode alcohol dehydrogenases, may be tested for use in engineering host strains of the invention that produce one or more branched-chain alcohols.

In some preferred embodiments, a recombinant photosynthetic microorganism includes a heterologous gene encoding a 2-ketoacid decarboxylase and a heterologous gene encoding an alcohol dehydrogenase, such as a branched-chain alcohol dehydrogenase. In some embodiments, a photosynthetic microorganism carries a heterologous gene encoding kdcA of Lactococcus lactis and a heterologous gene encoding adh6 of Saccharomyces cereviseae. In some embodiments, a photosynthetic microorganism carries a heterologous gene encoding pdc6 (also called pdc6) of Pichia stipitis and a heterologous gene encoding adh6 of Saccharomyces cereviseae. Recombinant photosynthetic microorganism that include a heterologous gene encoding a 2-ketoacid decarboxylase and a heterologous gene encoding an alcohol dehydrogenase can be used for the production of branched-chain alcohols such as one or more of isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol.

Additional enzymatic activities that catalyze particular reactions in the overall pathway may be provided by numerous polypeptides. For example, acetolactate synthase is an example of a designation for the enzyme that catalyzes the conversion of pyruvate to 2-hydroxy-2-methyl-3-oxobutanoate. Because enzymatic nomenclature various between organisms, it should be noted that the names provided above are merely illustrative of a class of enzymes that catalyze the particular steps of the pathway. The enzymes contemplated for use with the invention are those that catalyze the reactions illustrated and are not limited to the enzymatic names provided. In addition, homologs to these genes encoding desired enzymatic activities that are identified in genomic and metagenomic sequence databases can also be tested for activity and introduced into photosynthetic microorganisms. Heterologous polypeptide-encoding sequences are linked to appropriate gene expression regulatory elements (i.e., a promoter and terminator). Altered versions of these genes and homologs that have enhanced catalytic activity (e.g., lower substrate K_(m), reduced allosteric feedback inhibition, etc.) can also be generated by random or directed mutagenesis, and then introduced into photosynthetic microorganisms.

Exemplary polypeptides catalyzing the substrate to product conversions of reactions (1), (11), and (16) include acetolactate synthases (EC 2.2.1.6). A number of genes have been identified that encode acetolactate synthase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): P27818, Q41768, Q6K2E8, P09342, P14874, Q41769, Q7XKQ8, P09114, P27819, P17597, P37251, P42463, Q5KPJ5, Q6SSJ3, O19929, P08142, Q09129, O78518, P27696, Q04524, Q02137, O08353, Q57725, Q59498, P0A623, O33112, P0A622, P69683, P69684, Q1XDF6, P36620, P27868, Q7U5G1, P07342, P00892, P66947, P66946, O67703, O28555, P37252, P57320, O85294, Q89AP8, Q9TLY1, P00894, Q9MS98, O78451, P45260, Q02140, Q57625, O27492, Q59499, P65162, O33113, P65161, P51230, Q1XDQ7, P21622, Q55141, P57321, O85293, Q89AP7, Q9RQ65, P00893, P45261, P40811, P0ADG2, P0ADG1, P0ADG3, P0ADG0, P0ADF9, P0ADF8, and Q04789.

Exemplary polypeptides catalyzing the substrate to product conversions of reactions (2), (12), and (17) include ketol-acid reductoisomerases (EC 1.1.1.86). A number of genes have been identified that encode ketol-acid reductoisomerase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): Q05758, P38674, O82043, P84534, P78827, Q01292, P06168, Q81T69, Q73BA1, Q81G13, Q63DX9, Q6HLF4, Q8G6V2, Q9Z565, Q9UWX9, Q81S27, Q73A47, Q81F27, Q63CV4, Q6HKA1, Q8G6V1, Q9FBT8, Q97YJ9, Q97X13, B0CE35, A1TRT6, Q6F821, Q1ILZ3, A5G1L8, A1W6T4, A3N3E9, A6VLU1, A0KEM1, A4STE2, Q8UDV0, Q0VSBS, Q0AB89, A6TTL2, Q2IJB7, Q8YUM5, Q3MGX7, O67289, A8ERD8, O28294, A0JXZ6, A8I679, A1KAB7, Q5NXP4, A7Z7B9, A7GMU0, Q9K8E7, Q65GI7, A8FFW6, Q5WEN2, P37253, A1UT97, Q6G2T6, Q6FZ98, A9IW67, Q7VRM0, Q2KWH7, Q7WCP6, Q7W566, Q7VZU4, Q89G50, A5EPB5, A4YZA6, Q2YQN2, Q57CC7, A9M637, Q8YI21, A5VRC9, B0CHG8, Q8FZU1, P57655, O51888, Q89A20, Q9RQ55, Q9RQ47, Q9AQ96, Q9RQ51, Q9AQ97, Q9AQ98, Q9AQA0, Q9AQ99, Q1BUZ9, A0K937, Q0BDB6, A3MI87, A2S472, Q62IM0, A1V2K2, A3NT91, Q3JUC2, A3N7K4, Q63VP6, Q39ED2, Q2SZP8, A4JGE3, Q142I6, A4XIL7, A7ZEF1, A7GXQ8, A0RQ02, A7I0D4, A8FL53, A7H4H9, Q9PHN5, A1VYZ2, Q5HVD9, Q3AEQ7, Q9A6H4, B0T4Y1, A9WC26, Q3APC3, A1BES5, Q8KER7, Q1R092, Q7P0H9, A8ACS4, A5CPY6, B0RIN6, Q97MV0, A6LPX8, Q18C83, A0Q0E9, A9KQ65, A3DIE1, Q6NHN2, Q8FPX1, A4QDN4, Q57179, Q4JUN9, Q47BH8, Q3Z891, A5FR44, Q3ZXI2, Q1J0T2, Q9RU74, Q30ZD3, Q24XT5, A4J179, Q72CA6, A1VE41, A8LS39, A7ZTX6, P58256, A8A6N0, A1AHU6, Q0TAU6, Q8FBR2, B1IWC4, P05793, Q1R4G3, A4WG34, A7MQH1, Q6CZD1, Q2NAU8, Q0RDI8, Q2J6V2, Q5KWJ2, Q39W76, Q74BW9, A4IRH9, A5G7V3, Q7NH80, A9GZJ4, Q5FRY8, Q0BS26, Q4QMN4, A5UE34, A5UHH1, P44822, Q0I511, B0UWE8, Q2S9V9, A1WUW3, Q5V520, Q18GT4, Q17X66, Q7VGW6, Q1CUH1, Q9ZMA9, O25097, A4G4H8, Q0C516, A6SZZ1, Q28T07, A6W7N6, A6TGG1, Q02138, A2RKQ6, Q0YY8, Q6AEP2, Q04P56, Q056H8, Q72M00, Q8EYH2, P97115, Q03UU4, Q92A29, Q71Y36, Q8Y5S0, A0AK91, Q2W1G2, A0L626, Q65WK8, A1TZ11, Q11I83, Q8TJJ4, A7I5I8, Q46FY8, Q12Y03, Q605K9, Q1GZE9, Q2FM37, Q58938, Q8TX44, A2SR20, A4FYE4, A6VJW0, Q8PZ26, A3CXJ5, Q6LZH4, A2SHM0, A5UMJ9, Q2NI95, O27491, A0B5E0, A6USF9, B0JRP2, Q2RIS6, A0QJC6, Q59500, P65150, A1KMZ6, A4TE07, O33114, Q73VH7, A0QUX8, A3,PXP9, A1UE95, Q1BAR7, A5U713, P65149, A0PPY3, A1T6Y9, Q31RQ2, Q5F7E5, A9M177, Q9JTI3, Q9JYI2, A1KUZ8, Q0AGM0, Q82UZ3, Q1QJU8, Q2YBU1, Q3J875, A6Q461, Q3SQ46, Q5YRW2, Q2G623, Q8EN66, A6WZX5, A1B2W3, A7HVZ2, Q9CLF1, Q3B594, A1AS39, Q4FPQ6, A9BGP6, Q7MYK9, Q6LVZ5, A1VN04, Q12B50, A4SXR3, Q9F7L6, Q6A7Z2, A3PEE9, A2C482, A8G6C6, A2CB87, A2BY22, Q319H3, Q7VAR8, Q7V8M5, Q7V0F0, A2BSN6, Q46JF6, A4SFC5, Q48N66, A6VCE7, Q02FX9, Q9HVA2, Q1I4R5, Q4K608, A4XR11, A5W952, Q3K6T1, B0KHU2, Q88DZ0, Q888N4, Q4ZY66, A4VP66, A4VPI6, Q4FUB6, Q1QDE6, A5WD25, Q9UZ09, Q8ZTE1, A4WLK6, A3MX05, Q8U2A3, A1RRZ8, Q0KCU2, Q473V5, Q1LPX7, Q8XXN8, A9WP08, Q2K6M2, Q1MED4, Q98KM7, Q52955, Q7UKY0, Q21T70, O32414, Q2IUS3, Q07PJ7, Q6N869, Q211Z6, Q139A2, Q2RX71, A3PH14, Q3J5C0, A4WQ93, Q0S2H3, A7NJH8, Q168N1, Q1ARE4, A1AW99, Q21HN0, A4FMQ5, A8M5F9, A9MJN4, Q57HU2, Q5PJZ5, A9MXE7, Q2S0M9, Q8Z381, A4X4A5, P05989, A8GL54, Q8E9D5, Q31UL0, Q329V3, Q7UB34, Q3YVJ0, Q5LTP7, Q1GE81, A6UAW6, Q2NQA9, Q02CM4, A9GW78, Q1GT37, A5V3W3, A7X4M9, A6U3E1, Q2FF68, Q2FWK4, A5IUK2, Q2YUF3, Q5HEE5, A6QIQ2, P65151, P65152, Q6GF17, Q6G7Q2, A8Z4V9, P65153, Q5HMG0, Q8CRQ6, Q4L7T9, Q49Z11, Q59818, A8AVN4, Q8DW43, Q04M32, Q97SD7, Q8DR03, A4W3V8, A3CQ86, A4VXL3, Q5LXV0, Q5M2F2, Q03IJ9, Q9F0I7, Q4J8K9, Q30T61, Q971A9, Q31MY7, Q8DGR0, Q2JXL2, Q2JKN5, Q5N667, A5GMM6, Q7U5Q1, A5GRI8, Q0IC80, Q3AVC2, Q3ALC5, QOAV19, P29107, Q47SB6, Q9WZ20, A5IJM5, Q72JC8, Q5SJ03, Q8RDK4, Q31HZ1, Q3SHE4, Q8KTR6, Q10WP7, Q83HI9, Q83GP6, Q0W834, A5CWZ1, A5F449, Q9KVI4, Q5E1S3, Q87TN4, Q8DDC8, Q7MQH3, Q7M851, Q8PH09, Q3BPK3, Q4UYF7, Q8P5L5, Q2P757, Q5H4C1, Q9PCF9, Q87CM2, A1JI57, A7FD32, Q1CBS1, Q8ZAC2, Q1CNM0, A4TRD9, Q66G37, and Q9X5F8.

Exemplary polypeptides catalyzing the substrate to product conversions of reactions (3), (13), and (18) include dihydroxy-acid dehydratases (EC 4.2.1.9). A number of genes have been identified that encode dihydroxy-acid dehydratase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): Q10318, P39522, Q6FCR9, Q5P8J4, Q7WQA2, Q7WC98, Q7W069, Q89LK8, Q394V3, Q8FPX6, Q8TPV2, Q5Z0M2, Q3IJH1, Q475B2, Q98BZ8, Q49Z08, Q6F6Q0, Q5P6F1, Q7WJP7, Q7W497, Q7VUN6, Q89KY5, Q39DS9, Q8FMR1, Q8TKM8, Q5YX61, Q3ID04, Q46YI9, Q98LB3, Q49UX2, Q5NY71, Q7WFQ5, Q89HA2, Q5YRV8, B0CEN4, A1TMA7, Q1ILZ0, A0LSR8, A5FXD0, A3MYG9, A6VLE6, A0KQS4, Q9YG88, A4SHE9, Q8UE43, Q2IH17, A7HIA1, Q8YTE6, O67009, A8EWJ4, O29248, A0JXZ9, A8IES7, A1K344, A7Z5T7, A0RCL3, Q81S26, Q9XBI3, A7GNQ7, Q81F26, Q63CV3, Q5L9I8, Q64PS6, Q9K8E4, Q6HKA0, Q65IB0, A8FEC5, Q5WEM9, P51785, Q8A608, Q6G543, A9ILS3, A1A0T7, Q8G3H2, Q7VRL8, Q491Z0, Q2KZT7, Q2YNW9, Q57FS2, A9M6V2, Q8YEN0, A5VN43, B0CIL1, Q8G353, P57656, O51887, P59426, Q056W3, Q9RQ56, Q9RQ48, Q9RQ52, A3MLQ5, A2SAC7, Q62LG7, A1V5Z0, A3NSI6, Q3JV12, A3N6U9, Q63WB9, Q2T0B6, A4JN03, A4XHR9, A7GVT2, A0RRN7, A7I439, A8FJH6, A7H1A6, Q9PJ98, A1VX91, Q5HXE4, Q3AER0, P55186, A9WF68, Q3APB9, A1BES8, Q8KER4, Q1QU47, Q7NYJ7, A5CPY3, B0RIN3, Q97EE3, A6LTK6, A5N8V4, A0Q0E8, P31959, A9KT71, A3DIY3, Q47UN7, Q6NHN6, A4QDM9, Q8NQZ9, Q4JUN3, Q11NN5, Q47JC0, Q3Z888, A5FR35, Q3ZXH9, Q1IYZ8, Q9RV97, Q317H9, Q725Q1, A1V9E1, A8LKN5, A7ZTX3, Q8XAV1, A8A6M7, A1AHU3, Q0TAU9, Q8FBR5, B1IWX5, P05791, Q1R4G6, A4WG37, Q6CZC7, B0TZC0, Q2J4D1, A0Q6R5, Q5NH32, Q5KYA5, Q39W79, Q74BW7, A5G7V6, Q7NGK1, Q5FN26, Q0BQR6, Q4QMF8, A5UDY7, A5UHP2, P44851, Q0F1, B0UW18, Q2SA20, A1WTG1, Q5V545, Q7VHW3, A4G341, Q0C2B5, A8AB39, A6SVP5, A6TGF8, Q02139, Q02YY5, Q6AEN9, Q04RA5, Q053H5, Q72TC0, Q8F219, Q03UL2, Q92A32, Q71Y38, Q8Y5S2, A0AK88, Q2W485, A0L8Q3, Q65QD4, A6W1Q9, A6UUU2, A717L0, Q46AU2, Q12TW7, Q606D6, Q1H4H6, Q2FMZ1, Q58672, Q8TW40, A4FZM0, A6VIV7, Q8Q078, A3CU68, Q6M0F3, A2SFL0, A5UML4, A4YEN4, Q2NE10, O27498, A0B6Y9, A6URV4, B0JJP7, Q2RG93, A0QMH2, P65155, O06069, Q73TT7, A3PSS2, P65154, A0PMV4, Q3IMV2, Q5F8G6, Q9JUE0, Q9JS61, A1KU04, Q0ADX6, Q82XY7, Q1QRS7, Q2YC67, Q3J9N3, A6Q182, Q3SW60, A1SM84, Q2G7E9, Q8EN63, A6WV39, A6LDP0, A1B673, A7HXI4, P57957, Q3A3A5, Q3B589, A1AS43, Q4FM19, Q7MYJ5, Q6LLH7, Q6KZ30, A1VR98, Q12BW0, A3PCI2, A8G4F2, A2CAC6, A2BW57, Q31BA3, Q7VC95, Q7TV16, Q7V1T1, A2BQQ9, Q46LF6, A4SFC2, Q48PA6, Q15MY9, A6UYF6, Q02U62, Q9I6E0, Q1IGF7, Q4K498, A4Y036, A5WAG2, Q3K559, B0KN82, Q88CQ2, Q87V83, Q4ZZ83, A4VRN4, Q4FS54, A1SRU7, Q9UZ03, Q8ZYU6, A4WN46, A3MUK8, Q9U297, A1RSI5, Q8XWR1, Q8XWR1, A9WP05, Q2K9I7, Q1MIB2, Q92M28, Q7UJ69, P31874, Q21X56, Q2ISQ1, Q07IE7, Q6N9S5, Q2RTF9, A3PRB5, Q3IXP4, A7NNA3, A5UY13, A1AWH6, Q21NV7, A8M5F5, Q57HU7, Q5PK00, A9MXE2, Q8Z377, P40810, A1SAS5, A3D9T2, A6WTI9, A9L621, Q12IN9, Q088M9, B0TJR3, A3Q9L6, Q8E9D9, A8GZD9, A4YBI7, A0KS32, A8G0Z3, Q0HNC3, Q0HQG3, A1RPG2, Q31UL3, Q329V0, Q0SYW3, Q83PI6, Q3YVJ3, Q5LN98, Q1GDP8, A6UD23, Q2NQA6, Q1GTW7, A7X4M5, A6U3D8, Q2FF71, Q2FWK7, A5IUJ9, Q2YUF6, Q5HEE8, A6QIQ0, P65156, P65157, Q6GF19, Q6G7Q4, A8Z4V6, P65158, Q5HMG3, Q8CNL6, Q4L7T6, Q82E99, O69198, Q8DRT7, Q04I44, P65159, P65160, A4W3W3, A3CR42, A4VXL9, Q5LYH1, Q5M334, Q4J860, Q30UI5, A6QD02, Q97UB2, Q96YK0, Q67KX6, Q2LXP6,Q31QL1, Q8DK13, A0LF54, Q2JTX6, Q2JK60, Q5N3N2, Q7U763, A5GTE2, Q3AXL0, Q3AK67, P74689, Q47MS7, Q9WZ21, A5IJM4, Q72JA8, Q5SIY0, Q8RDJ9, Q31I28, Q8KTS9, Q11AD1, Q83HI6, Q83GP9, A1WMU5, A5F497, Q9KVW0, Q5E1P2, Q87KB6, Q8DDG1, Q7MGI8, Q7MAN4, Q8PQI0, Q3BYS5, Q4UZT2, Q8PDJ3, Q2NY76, Q5GUY8, Q9PH47, Q87F63, A1JI53, A7FD26, Q1CBS9, Q8ZAB3, Q1CNM8, A4TRE8, Q66G45, and Q5NLJ4.

Exemplary polypeptides catalyzing the substrate to product conversions of reaction (6) include citramalate synthases (EC 2.3.1.182). A number of genes have been identified that encode citramalate synthase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): Q8TJJ1, Q58787, Q8TYB1, P58966, and O26819.

Exemplary polypeptides catalyzing the substrate to product conversions of reaction (7) include 2-methylmalate dehydratases (EC 4.2.1.35). A number of genes have been identified that encode 2-methylmalate dehydratase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): P81291 and Q58673.

Exemplary polypeptides catalyzing the substrate to product conversions of reactions (8), (20), and (21) include 3-isopropylmalate dehydratases (EC 4.2.1.33). A number of genes have been identified that encode 3-isopropylmalate dehydratase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): O28316, Q89X98, Q9RTY9, Q65VS0, Q8TLF1, Q8TVF2, Q8PZT3, O27439, Q9UZ07, Q8U2A1, Q1AZC4, Q57TE8, P15717, Q9WYC7, O28084, Q89X34, Q9RTI6, Q65V07, Q8TQZ3, Q8TW29, Q8PUG1, O27668, Q9V1J0, Q8U0C0, Q1AVC5, Q57SN1, Q8ZRJ0, Q9WZ24, B0CG35, A1TLH6, Q6FEW0, Q1MI3, A3M1S8, A0LVA3, A5G0G6, A1WAS7, A3MYL1, A6VQL0, Q44427, A0KGM7, A4SR64, Q8UBY9, Q0VPI0, Q0A9B0, Q2IJC2, A7HBI2, Q8YX02, Q3M614, O67078, A8EQZ0, A1R7K0, A0JXX8, Q74ZM9, A1K4A1, Q5P1J8, P96195, A7Z7B6, A0RBL4, Q81T66, Q73B98, A7GMU3, Q81G10, Q63DX6, Q5LAB1, Q64QP1, Q9K8F0, Q6HLF1, Q65GJ0, A8FFW3, Q5WEN5, P80858, Q8A6L7, A6L1V8, A0ZZS7, Q8G4W2, Q7VQJ8, Q493R2, Q7WKH6, Q7W931, Q7VY75, Q2YLP7, Q57AZ0, A9M8P2, Q8YJC9, A5VSN3, B0CIF7, Q8FYG9, P56934, O85065, P59519, Q5WPZ8, O85072, Q9EVG2, P58945, P48573, O31293, Q9EVG5, Q9EVE0, Q9EVH4, Q9EVH7, Q9AQC6, Q9EVG8, Q9EVI6, Q9EVI0, Q9EVI3, Q1BM55, A0AZ60, Q0BAC8, A3MBT5, A2S127, Q62AI6, A1UZ32, A3P7N9, Q3JKG6, A3NM77, Q63JK9, Q393X2, Q2T7H8, A4JMB6, A8MDY8, A4XJ48, A7ZFP0, A7H0L8, A0RMG7, A7HZP6, A8FP33, A7H665, Q9PLW1, A1W1X0, Q5HS78, Q00464, Q9ABN0, Q7NUB6, A8ALM7, Q97EE0, A6LPX4, Q18AJ2, A5MZ75, A9KT79, A3DHI4, Q47WG2, Q6NHL0, Q8FPR3, A4QDS8, P58946, Q4JUX2, Q11NN8, Q47HR4, Q30WD3, Q24XT4, Q726X4, A1VAE7, A8LKJ1, A7ZHG4, Q8XA00, A7ZW23, A1A7C0, Q0TLR7, P0A6A7, B1IRA6, P0A6A6, Q1RGC5, A4W6H7, A7MIC7, Q6D0G6, A5FKC6, Q0RDK7, Q2J6W9, Q5KWJ5, Q39W70, Q74BX5, A4IRH6, A5G7U4, Q7NFV7, Q5FUG3, Q0BRH4, Q4QLS2, A5UD83, P44968, Q0I2G3, B0USF4, Q2SJD8, A1WY14, Q5V518, Q7ZAG7, Q7VH31, A4G4F5, A6T015, Q28W60, A6T4L7, Q02142, A2RKR3, Q02YY1, Q6AFK7, Q72RC4, Q8F4E6, Q03UM2, Q92A26, Q71Y33, Q8Y5R7, A0AK94, Q2VZV4, A0L8J4, A1U0Y0, Q0AT09, A6VX34, Q11CQ6, A7IA28, Q606F2, Q1H0L4, P81291, A4YF03, B0JY97, Q2RG98, A0QJB7, Q7TXH6, A1KMY2, A4TE23, O33123, Q73VI7, Q938C9, A3PXR2, A1UEA8, Q1BAQ4, A5U6Z9, O53237, A0PPZ6, A1T700, Q3IRQ4, Q5F8T1, A9LZF7, Q9JU82, Q9JZI5, A1KU80, Q82WI9, Q1QHI4, Q3J716, A6Q5L6, Q3SNV3, Q5YRY0, A1SLW5, Q2G958, Q8EN69, A6WXG4, Q04DA3, A1B513, A7HT10, Q9CJN7, Q4FP15, Q7N127, Q6LV26, P18250, Q6L0K5, A1VRR0, Q126M9, A4SWW8, A3PAX7, A2C088, A8G2R6, A2CCJ0, A2BUN7, Q31CS6, Q7VDT0, Q7V4U5, Q7V336, A2BP55, Q46HB8, Q48K99, Q15QR2, A6V2V3, Q02PT4, Q9HZA3, Q1ICW0, Q4KF08, Q3IJS4, A4XVW2, B0KF81, Q88LE8, Q884C2, Q4ZUZ6, A4VKE7, Q4FRU7, Q1QAF2, A5WGT5, Q8ZW41, A4WMI6, A3MWJ4, O59391, A1RVH3, Q5JFV6, Q46YV7, Q44023, Q8XXX3, Q2K376, Q8VMA6, Q1MAJ9, Q98EF1, Q92L76, P55811, P55251, P17279, Q7UIA7, Q2J3A1, Q07UU3, Q6ND69, Q21CT4, Q13DU7, Q2RV55, A3PMQ9, Q3IZI8, A4WNV5, A7NJH2, Q16CH7, A5UUP5, A1AW36, Q21IY3, A4FMP2, A8M5I3, A9MQE0, Q5PDG3, Q2S0M6, Q8Z9I2, A4X4C8, O14289, A8G9Q9, A1S2E2, A3CZK7, A6WIB7, A9KY15, Q07WH1, B0TQM2, A3QIN8, Q8E9N4, A8H999, A4Y2M2, A0L1Q8, A8FQ85, Q0HE67, Q0HZT2, A1REY2, Q326G3, Q32K22, Q0T8C6, Q821C2, Q3Z5T8, Q5LX06, Q1GDM6, A6UE05, Q2NVW5, Q01Z81, A9GL99, Q1GR84, A7X4N3, A6U3E4, Q2FF65, Q2FWK1, A5IUK5, Q2YUF0, Q5HEE2, A6QIQ5, P63435, P63436, Q6GF14, Q6G7P9, A8Z4W2, P58947, Q5HMF7, Q8CNL1, Q4L7U2, Q49Z14, Q82JR8, O86534, A8AWP5, Q9AIM3, Q8DTG4, A3CMJ2, Q5LZF4, Q5M406, Q03KB3, Q4JC09, Q30NZ0, A6Q6J8, Q97VY2, Q974R0, Q67MJ2, Q2LWJ2, Q31LZ1, Q8DKF0, Q2JQU3, Q2JPG2, Q5MZY3, A5GIG7, Q7U9J4, A5GWH5, Q0IDDS, Q3AW21, Q3AN00, P54384, Q47SA3, Q72JB3, Q9ZND5, Q8RDK2, Q31HI2, Q3SHL1, P49601, A5CX27, A5F5E2, Q9KP81, Q5E858, A7MWC3, Q87SS9, Q8DED9, Q7MP79, Q7M9Z9, P58948, Q3BPJ6, Q4UYG5, P58949, Q2P765, Q5H4D1, Q9PAX0, Q87BP9, P07264, A1JJH5, A7FM86, Q1C1Z4, Q8ZIH0, Q1CMP7, A4TQA4, Q66EM3, Q5NRC5, O29626, Q7WKH5, Q7W930, Q7VYI1, Q7NW22, Q9RTY5, Q65VR9, Q8TJM9, Q8TX94, Q8PZ49, O26917, Q9UZ06, Q8U2A0, Q57TE9, P04787, Q9WYC8, O28513, Q7WIN3, Q7W749, Q7VY74, Q7NUB8, Q9RTI0, Q65V08, Q8TU71, Q8TW31, Q8PWT6, O27440, Q9V1I9, Q8U0B9, Q57SN0, Q8ZRI9, Q9WZ25, Q8TRF7, A1TLH5, Q6FEV8, Q1MI4, A3M1S9, A0LVA2, A1WAT0, A3MYL0, A6VQL1, Q44428, A0KGM6, A4SR65, Q8UBR0, Q0VPI1, Q0A9A9, Q2IJC3, A7HBI3, Q8YX03, O67399, A8ETJ8, A1R7J9, A0JXX7, A8ILN3, A1K4A2, Q5P1J7, P96196, A7Z7B5, Q81T65, Q73B97, A7GMU4, Q81G09, Q63DX5, Q9K8F1, Q6HLF0, Q65GJ1, A8FFW2, Q5WEN6, P94568, A0ZZS8, Q8G4W1, Q7VQJ9, Q493R3, Q2KYL4, Q89X27, A5E8Z8, Q2YL51, Q579A4, A9MCG4, P65275, A9WVP8, P65276, P56935, O85066, P59516, O85073, Q9ZEZ5, P59019, P48574, O31294, Q0BAC6, A3MBT7, A2S129, Q62AI8, A1UZ33, A3P7N7, Q3JKG8, A3NM75, Q63JL1, Q2T7H7, A4XJ49, A8FP32, A7H664, Q9PLW2, A1W1W9, Q5HS79, Q9ABN1, B0T3B7, A9WC30, Q1QUQ9, A8ALM8, A5CQN3, B0REY1, Q97EE1, Q18AJ1, P31960, Q47WG1, Q6NHK9, Q8FPR2, A4QDS9, Q8NQV7, Q4JUX3, Q11NN7, Q47HR2, Q30WD2, Q726X3, A1VAE6, A8LKI9, A7ZHG3, Q8XA01, A7ZW22, A1A7B9, Q0TLR8, Q8FL79, B1IRA7, P30126, Q1RGC6, A4W6H6, A7MQ53, Q6D0G5, Q2NC39, Q0RDK8, B0TW84, Q2J6X0, A7NES0, Q2A1A1, A0Q406, Q0BK19, Q5KWJ6, A9HS54, Q5FUG4, Q0BRH5, A0M367, Q4QLS1, A5UD82, A5UIC6, P44438, Q0I2G4, B0USF3, Q2SJD7, Q7VH32, Q28W72, A6W7Q8, A6T4L6, Q02144, A2RKR1, Q6AFK6, Q04RN8, Q051Y3, Q72RC5, Q8F4E5, Q03UM3, Q92A25, Q71Y32, Q8Y5R6, A0AK95, Q2VZV3, A0L8J5, A1U0X9, A6VX35, Q606F3, Q8RP98, Q1H0L5, Q58673, A2SHT0, A4YF02, Q2RG99, A0QJB6, P65278, A1KMY1, A4TE22, O33124, Q73VI8, Q938C8, A3PXR3, A1UEA9, Q1BAQ3, A5U6Z8, P65277, A0PPZ7, A1T701, Q5F8T3, Q50974, P50181, A9LZF9, Q9JU81, Q9JZI6, A1KU82, Q0AGY1, Q82WI7, Q1QHH8, Q3JCC5, Q3SNU7, Q5YRY1, A1SLW4, Q2G955, Q8EN70, A6X449, Q04DA2, Q9LCR5, A1B515, A7HT11, Q9CJN8, Q4FP16, Q7N126, Q6LV27, A1VRR1, Q126M8, A4SWX0, Q48K98, A6V2V4, Q02PT3, Q9HZA4, Q1ICV9, Q4KF07, Q3US5, A4XVW1, B0KF82, Q88LE7, Q884C1, Q4ZUZ5, A4VKE8, Q4FRU8, Q1QAF3, A1SVE8, A5WGT4, Q8ZW36, A3MWK3, O59393, A1RVI0, Q5JFV7, Q46YV9, Q44022, Q8XXX4, Q2K2V2, Q1MA52, Q98E51, Q92LA1, Q7UIA6, Q21XI0, Q2J3A7, Q07UT6, Q6ND74, Q21CT1, Q13DV3, Q2RV54, A3PMQ7, Q3IZJ0, A4WNV6, A7NJH1, Q16DI8, A5UUP4, Q1AZC3, A1AW35, Q21IY4, A4FMP1, A8M5I4, A9MQE1, Q5PDG4, Q8Z9I3, A4X4C9, A8G9Q8, A1S2E3, A3CZK8, A6WIB8, A9KY16, Q12SE4, Q07WH2, B0TQM3, A3QIN7, Q8E9N5, A8H998, A4Y2M3, A0L1Q7, A8FQ86, Q0HE68, Q0HZT1, A1REY3, Q326G4, Q32K23, Q0T8C7, P59714, Q3Z5T9, Q5LX07, Q1GDM5, A6UDX5, Q2NVW6, Q01Z80, Q1GR82, A7X4N4, A6U3E5, Q2FF64, Q2FWK0, A5IUK6, Q2YUE9, Q5HEE1, A6QIQ6, P65279, P65280, Q6GF13, Q6G7P8, A8Z4W3, Q8NVI9, Q5HMF6, Q4L7U3, Q49Z15, Q82JR9, O86535, A8AWP6, Q9AIM2, Q8DTG5, Q97QF9, A4W420, A3CMJ3, A4VXS3, Q5LZF5, Q5M407, Q03KB4, Q4JC10, Q30RK1, A8Z5R9, Q97VY3, Q974Q9, Q67MJ3, Q2LWJ1, P74207, Q47SA2, Q72JB2, Q9ZND4, Q8RDK1, Q3SHL2, A5CX59, A5F5E3, Q9KP80, Q5E859, A7MWB9, Q87ST0, Q8DED8, Q7MP80, Q7M887, Q8PH04, Q3BPJ7, Q4UYG4, Q8P5K9, Q2P764, Q5H4D0, Q9PAX1, Q87BQ0, A7FM87, Q1C1Z3, Q8ZIH1, Q1CMP8, A4TQA5, Q66EM4, and Q5NRC4.

Exemplary polypeptides catalyzing the substrate to product conversions of reactions (9) and (22) include 3-isopropylmalate dehydrogenases (EC 1.1.1.85). A number of genes have been identified that encode 3-isopropylmalate dehydrogenase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): Q9SA14, Q7WNM3, Q89XA0, Q47HR1, P93832, Q7WKH4, Q89X19, Q479H7, Q9FMT1, P87256, P87257, Q6FEV6, P24404, Q2IJK7, Q8YXA2, Q3M8T9, O66607, O29627, Q8NKB8, O60027, Q5P1J6, P96197, Q81T67, Q73B99, P05644, P12010, Q81G11, Q63DX7, Q5LAB4, P54354, Q9K8E9, Q6HLF2, Q65GI9, P41019, Q5WEN4, P05645, Q8A6M0, Q8G500, Q7VQJ7, Q493R1, Q2KYL5, Q7W929, Q7VY73, P29102, Q2YL58, Q579B1, Q8YCX4, Q8FVF3, P56933, O85064, P59515, Q5WPZ9, O85071, Q9EVG3, P59027, P48572, O31292, Q9EVG6, Q9EVE1, Q9EVH5, Q9EVH8, Q9AQC8, Q9EVG9, Q9EVI7, Q9EVI1, Q9EVI4, Q845W3, Q62AI9, Q3JKG9, Q63JL2, Q393X4, Q2T7H6, Q9PLW0, Q5HS77, P87186, Q01987, O14429, P07139, Q6PY58, Q9HDQ5, Q3AEQ2, Q9ABN3, Q12545, Q3APC4, P59028, Q1QUR0, Q7NUC2, A5CPZ4, B0RIP4, Q97EE2, P31958, Q47WG3, Q6NHM7, Q8FPV5, A4QDP9, P94631, Q4JUQ0, Q6B458, Q3Z896, Q3ZXI7, Q1IZK2, Q9RTH9, Q30WD0, Q24XT2, Q6ANR1, Q726X1, Q8X9Z9, Q8FL76, P30125, Q1RGC4, Q6D0G7, Q2J6V8, Q5KWJ4, Q39Y29, Q748X2, Q7NFH4, Q5FUG5, Q4QLS3, P43860, Q2SJD6, Q9HDQ1, Q7VH33, Q28W67, P23390, P41766, Q02143, Q6AEP6, Q72RH7, P24015, Q92A27, Q71Y34, Q8Y5R8, Q2VZV2, Q65V05, Q606F4, Q58130, O27441, Q2RGA0, A0QJC2, P94929, A1KMY9, A4TE12, Q9Y897, O33117, Q73VI1, A3PXQ2, A1UE98, Q1BAR4, A5U706, P95313, A0PPY6, A1T6Z4, Q5F8T6, P50180, Q9JU79, Q9JZI9, P34738, Q82WI6, Q2Y7Q8, Q3JCC4, Q3SNU3, Q5YRX2, Q2G4X5, Q8EN68, Q9CJN6, Q3A3B2, Q3B595, Q4FP17, O59930, Q7N128, Q6LV25, P34733, P08791, O94114, Q31B91, Q7VC80, Q7V842, Q7V1R9, Q46LE2, Q48K97, Q51375, Q4KF05, Q3IJS3, Q3KF21, Q88LE5, Q884C0, Q4ZUZ4, Q4FRV0, Q1QAF5, Q46YW0, Q1LKH7, Q8XXX5, Q2K2V0, Q1MA50, Q98E57, Q92KY8, Q7UIE1, Q21XI1, Q2J3B4, Q6ND82, Q21CS1, Q2RV53, Q3IZJ3, Q0S2H1, Q21IY5, A4FMQ2, Q96WT9, Q57TE7, Q5PDG2, Q2S0M8, Q8Z9I1, P37412, P18869, Q8E9N3, Q326G2, Q32K21, Q83SP1, Q3Z5T7, Q5LWZ5, Q2NVW4, P29696, Q6TWC4, Q00412, Q2FF66, Q2FWK2, Q2YUF1, Q5HEE3, P65100, P65101, Q6GF15, Q6G7Q0, Q8NVJ0, Q5HMF8, Q8CNL2, Q4L7U1, Q49Z13, Q82JN6, O86504, Q8DTG3, Q8DPJ4, Q5LZF3, Q5M405, Q30RK2, Q9UXB2, P50455, Q67JY2, Q31N34, P59029, Q2JTN8, Q2JL30, Q5MZ40, Q7U840, Q3AYS1, Q3AIH4, P73960, P24098, Q47SB4Q9WZ26, P61494, Q5SIY4, P61495, Q8RDK0, Q31HI0, Q3SHL3, Q56268, Q9KP82, Q5E857, Q87SS8, Q8DEE0, Q7MP78, Q7M886, Q8PH05, Q3BPJ8, Q4UYG2, Q8P5L1, Q2P762, Q5H4C7, Q9PAX3, Q87BQ1, P41926, P18120, P04173, Q8ZIG9, Q66EM2, Q9P3Y0, Q96W10, and Q5NPQ9.

Exemplary polypeptides catalyzing the substrate to product conversions of reaction (10) include threonine ammonia-lyase (EC 4.3.1.19). A number of genes have been identified that encode threonine ammonia-lyase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): P09367, P25379, Q9ZSS6, Q9KC63, P37946, P53607, Q39469, Q04513, P04968, P46493, Q02145, P66898, Q9X7F1, P66897, Q9CKJ2, P20506, P25306, P31212, Q2FF63, Q2FWJ9, Q2YUE8, Q5HEE0, Q99SJ1, Q7A4H2, Q3V7T5, Q3V7T4, Q8NVI8, Q5HMF5, Q8CNK9, Q4L7U4, Q49Z16, P0AGF8, P0AGF7, P0AGF6, P11954, P0AGF9, Q2FH01, Q2FYJ3, Q2YY67, Q5HFY5, Q99U50, Q7A5L8, Q6GGX0, Q6G9C4, Q8NWQ4, O42615, O94634, P00927, and P55664.

Exemplary polypeptides catalyzing the substrate to product conversions of reaction (19) include 2-isopropylmalate synthases (EC 2.3.3.13). A number of genes have been identified that encode 2-isopropylmalate synthase enzymes that can be tested for appropriate activity and introduced into photosynthetic microorganisms, including, but not limited to, the genes that encode the following enzymes (UniProtKB Accession numbers): Q9LPR4, O29305, Q8TKQ6, Q57926, Q8TW28, P58967, O27667, Q9UZ08, Q8XXP1, Q97ZE0, Q974X3, Q8RDK3, Q9C550, O30020, Q8THA5, Q58595, Q8TYM1, P58968, O27525, Q9V1J1, Q8XSZ5, Q97W36, Q971S5, Q8RCF9, O04973, O04974, Q8UD63, P48575, O67862, A0JX36, Q81T68, Q9K8E8, Q8RL85, P94565, Q7VQJ6, Q89GB0, Q8YIJ3, Q8FZC4, Q9ZEY8, O85063, Q89A49, Q5WQ01, O85070, Q9EVG4, P58898, P48571, O31287, Q9EVH6, Q9EVE3, Q9EVH0, Q9EVI8, Q9PLV9, Q9A823, Q7P0H2, Q97MC5, Q8FU05, A4QAP0, P42455, P85362, Q9RUA9, Q8X9Z8, Q8FL75, P09151, Q7NI93, P43861, Q7VH30, A6WDF2, Q02141, Q72RL9, Q8F445, Q92A28, Q71Y35, Q8Y5R9, P94907, Q7TVV6, Q9CB76, P96420, A0PVE6, Q9JUK6, Q9JZG1, Q820M0, Q8EN67, Q9CJN5, Q7N129, Q7VBG1, Q7TUV5, Q7V121, Q48LY5, A6V0X2, Q9HXK5, Q1I5K2, Q4K6V7, A4XY24, A5VZB6, Q3K7C3, B0KRD9, Q88P28, Q886Y1, Q4ZX14, A4VNV6, Q8ZW35, Q8U2A2, O59390, Q1MDH6, Q98HN3, Q9X7L2, Q7UI51, Q8Z9I0, P15875, O59736, Q8E9N2, Q83SP0, Q39891, Q5HEE4, P63476, P63477, Q6GF16, Q6G7Q1, P58899, Q5HMF9, Q8CNL3, Q4L7U0, Q49Z12, Q82BV3, O31046, Q8DJ32, Q7U892, P48576, Q9WZ23, Q56216, Q9KP83, Q87SS7, Q8DEE1, Q7MP77, Q7M9W4, P58900, P58901, Q9PCG3, Q87CL8, P06208, Q8ZIG8, Q66EM1, and Q12166.

Host Organisms

The host cells used to prepare the cultures of the invention include cells of any photosynthetic microorganism which is able to convert inorganic carbon into a substrate that is in turn converted to branched-chain alcohols. These organisms include prokaryotes as well as eukaryotic organisms such as algae and diatoms. Carbon dioxide (which, along with carbonic acid, bicarbonate and/or carbonate define the term “inorganic carbon”) is converted to a reduced carbon molecule in the photosynthetic process. An inorganic carbon source can be used to supply inorganic carbon to the photosynthetic microorganism, in which the inorganic carbon source includes any way of delivering inorganic carbon, optionally in admixture with any other combination compounds which do not serve as the primary carbon feedstock, but only are present as a mixture or carrier (for example, emissions from biofuel (e.g., ethanol) plants, power plants, refineries, as well as atmospheric sources).

Host organisms include eukaryotic microalgae and cyanobacteria (blue-green algae). Representative algae include green algae (chlorophytes), red algae, diatoms, prasinophytes, glaucophytes, chlorarachniophytes, euglenophytes, chromophytes, and dinoflagellates. A number of cyanobacterial species are known and have been manipulated using molecular biological techniques, including the unicellular cyanobacteria Synechocystis sp. PCC6803 and Synechococcus elongatus PCC7942, whose genomes have been completely sequenced.

The following genera of cyanobacteria may be used: one group includes Chamaesiphon, Chroococcus, Cyanobacterium, Cyanobium, Dactylococcopsis, Gloeobacter, Gloeocapsa, Gloeothece, Microcystis, Prochlorococcus, Prochloron, Synechococcus, and Synechocystis. Another group includes: Chroococcidiopsis, Cyanocystis, Dermocarpella, Myxosarcina, Pleurocapsis, Stanieria, and Xenococcus. Still another group includes Arthrospira, Borzia, Crinalium, Geitlerinema, Halospirulina, Leptolyngbya, Limnothrix, Lyngbya, Microcoleus, Oscillatoria, Planktothrix, Prochlorothrix, Pseudanabaena, Spirulina, Starria, Symploca, Trichodesmium, and Tychonema. Still another group includes Anabaena, Anabaenopsis, Aphanizomenon, Calothrix, Cyanospira, Cylindrospermopsis, Cylindrospermum, Nodularia, and Nostoc; and another group includes Chlorogloeopsis, Fischerella, Geitleria, Nostochopsis, Iyengariella, Stigonema, Rivularia, Scytonema, and Tolypothri.

In addition, various algae, including diatoms and green algae can be employed. Eukaryotic microalgae that can be used in the methods of the invention can include, but are not limited to, Achnanthes, Amphiprora, Amphora, Ankistrodesmus, Asteromonas, Boekelovia, Borodinella, Botryococcus, Bracteococcus, Chaetoceros, Carteria, Chlamydomonas, Chlorococcum, Chlorogonium, Chlorella, Chroomonas, Chrysosphaera, Cricosphaera, Crypthecodinium, Cryptomonas, Cyclotella, Dunaliella, Ellipsoidon, Emiliania, Eremosphaera, Ernodesmius, Euglena, Franceia, Fragilaria, Gloeothamnion, Haematococcus, Halocafeteria, Hymenomonas, Isochrysis, Lepocinclis, Micractinium, Monoraphidium, Nannochloris, Nannochloropsis, Navicula, Neochloris, Nephrochloris, Nephroselmis, Nitzschia, Ochromonas, Oedogonium, Oocystis, Ostreococcus, Pavlova, Parachlorella, Pascheria, Phaeodactylum, Phagus, Platymonas, Pleurochrysis, Pleurococcus, Prototheca, Pseudochlorella, Pyramimonas, Pyrobotrys, Scenedesmus, Skeletonema, Spyrogyra, Stichococcus, Tetraselmis, Thalassiosira, Viridiella, or Volvox species.

Transformation of Host Organisms

Photosynthetic microorganisms can be transformed by any suitable methods, including, as nonlimiting examples, natural DNA uptake (Chung, et al. (1998) FEMS Microbiol. Lett. 164: 353-361; Frigaard, et al. (2004) Methods Mol. Biol. 274: 325-40; Zang, et al. (2007) J. Microbiol. 45: 241-245), conjugation, transduction, glass bead transformation (Kindle, et al. (1989) J. Cell Biol. 109: 2589-601; Feng, et al. (2009) Mol. Biol. Rep. 36: 1433-9; U.S. Pat. No. 5,661,017), silicon carbide whisker transformation (Dunahay, et al. (1997) Methods Mol. Biol. (1997) 62: 503-9), biolistics (Dawson, et al. (1997) Curr. Microbiol. 35: 356-62; Hallmann, et al. (1997) Proc. Natl. Acad. USA 94: 7469-7474; Jakobiak, et al. (2004) Protist 155:381-93; Tan, et al. (2005) J. Microbiol. 43: 361-365; Steinbrenner, et al. (2006) Appl Environ. Microbiol. 72: 7477-7484; Kroth (2007) Methods Mol. Biol. 390: 257-267; U.S. Pat. No. 5,661,017) electroporation (Kjaerulff, et al. (1994) Photosynth. Res. 41: 277-283; Iwai, et al. (2004) Plant Cell Physiol. 45: 171-5; Ravindran, et al. (2006) J. Microbiol. Methods 66: 174-6; Sun, et al. (2006) Gene 377: 140-149; Wang, et al. (2007) Appl. Microbiol. Biotechnol. 76: 651-657; Chaurasia, et al. (2008) J. Microbiol. Methods 73: 133-141; Ludwig, et al. (2008) Appl. Microbiol. Biotechnol. 78: 729-35), laser-mediated transformation, or incubation with DNA in the presence of or after pre-treatment with any of poly(amidoamine) dendrimers (Pasupathy, et al. (2008) Biotechnol. J. 3: 1078-82), polyethylene glycol (Ohnuma, et al. (2008) Plant Cell Physiol. 49: 117-120), cationic lipids (Muradawa, et al. (2008) J. Biosci. Bioeng. 105: 77-80), dextran, calcium phosphate, or calcium chloride (Mendez-Alvarez, et al. (1994) J. Bacteriol. 176: 7395-7397), optionally after treatment of the cells with cell wall-degrading enzymes (Perrone, et al. (1998) Mol. Biol. Cell 9: 3351-3365). Agrobacterium-mediated transformation can also be performed on algal cells, for example after removing or wounding the algal cell wall (e.g., WO 2000/62601; Kumar, et al. (2004) Plant Sci. 166: 731-738). Biolistic methods are particularly successful for transformation of the chloroplasts of plant and eukaryotic algal species (see, for example, Ramesh, et al. (2004) Methods Mol. Biol. 274: 355-307; Doestch, et al. (2001) Curr. Genet. 39: 49-60; U.S. Pat. No. 7,294,506; WO 2003/091413; WO 2005/005643; and WO 2007/133558, all incorporated herein by reference in their entireties).

In some preferred embodiments of the invention, a gene encoding an enzyme that participates in a pathway that leads to the synthesis of a branched-chain alcohol (such as an enzyme disclosed herein), is cloned into an expression vector for transformation into an alga or photosynthetic bacterium. The vector includes sequences that promote expression of the transgene of interest, such as a promoter, and, where the engineered host strain is a eukaryotic microalga, may optionally include a transit peptide-encoding sequence for directing the expressed enzyme to the chloroplast of transformed eukaryotic cells, an intron sequence, a sequence having a polyadenylation signal, etc. Alternatively, if the vector does not contain a promoter in operable linkage with the gene of interest, the gene can be transformed into the cells such that it becomes operably linked to an endogenous promoter by homologous recombination or vector integration.

In some embodiments, a vector is designed for integration of the heterologous nucleic acid sequence into the host genome. For example, vectors can be: 1) targeted for integration into an algal or cyanobacterial chromosome by including flanking sequences that enable homologous recombination into the chromosome, 2) targeted for integration into endogenous host plasmids by including flanking sequences that enable homologous recombination into the endogenous plasmids, or 3) designed such that the expression vectors replicate within the chosen host.

Artificial chromosome vectors can also be used for the transformation of photosynthetic microorganisms when more than one gene that encodes an enzyme that participates in the synthesis of a branched-chain alcohol is transformed into an organism.

In some cases in which the nucleus of a eukaryotic host organism is transformed, it may be advantageous to include a sequence encoding a chloroplast transit peptide in the heterologous gene construct. The transit peptide sequence can be derived from a gene endogenous to the host organism, or can be derived from a gene from another species.

In some cases in which it may be advantageous to transform the chloroplast of a eukaryotic alga, vectors can be designed to have regions of sequences flanking the transgene (e.g., a 2-ketoacid decarboxylase gene) that are homologous to chloroplast sequences to promote homologous recombination and integration of the sequence of interest. In these embodiments, the vector preferably includes a promoter for expressing the transgene, in which the promoter functions in the chloroplast.

Vectors designed for expression of a gene in microalgae can in some embodiments include a promoter active in microalgae operably linked to the exogenous gene being introduced. A variety of gene promoters and terminators that function in green algae can be utilized in expression vectors, including, but not limited to promoters and terminators from Chlamydomonas and other algae (see, for example, Plant Cell Physiol 49: 625-632 (2008)), promoters and terminators from viruses, and synthetic promoters and terminators. Expression constructs can also optionally include an intron, such as an intron sequence from the host organism inserted into the exogenous gene, for optimal expression of the gene in the host.

For transformation of diatoms, a variety of gene promoters that function in diatoms can be utilized in these expression vectors, including, but not limited to: 1) promoters from Thalassiosira and other heterokont algae, promoters from viruses, and synthetic promoters. Promoters from Thalassiosira pseudonana that would be suitable for use in expression vectors include an alpha-tubulin promoter, a beta-tubulin promoter, and an actin promoter. Promoters from Phaeodactylum tricornutum that would be suitable for use in expression vectors include an alpha-tubulin promoter, a beta-tubulin promoter, and an actin promoter. The terminators associated with these genes, other diatom genes, or particular heterologous genes can be used to stop transcription and provide the appropriate signal for polyadenylation.

In some instances it can be advantageous to express a heterologous enzyme at a certain point during the growth of the transgenic host to minimize any deleterious effects on the growth of the transgenic organism and/or to maximize production of the branched-chain alcohol. In these instances one or more exogenous genes introduced into the transgenic organism can be operably linked to an inducible promoter. The promoter can be a lac promoter, a tet promoter (e.g., U.S. Pat. No. 5,851,796), a hybrid promoter that includes either or both of portions of a tet or lac promoter, a hormone-responsive promoter (e.g., an ecdysone-responsive promoter, e.g., U.S. Pat. No. 6,379,945) a metallothionien promoter (U.S. Pat. No. 6,410,828), or a pathogenesis-related (PR) promoter that can be responsive to a chemical such as, for example, salicylic acid, ethylene, thiamine, or BTH (U.S. Pat. No. 5,689,044). An inducible promoter can also be responsive to light or dark (U.S. Pat. No. 5,750,385, U.S. Pat. No. 5,639,952) or temperature (U.S. Pat. No. 5,447,858; Abe, et al., Plant Cell Physiol. 49: 625-632 (2008); Shroda, et al., Plant J. 21: 121-131 (2000)), or copper level (Surzycki, et al., Proc Natl Acad Sci USA. 104: 17548-17553 (2007)). The foregoing list is exemplary and not limiting. The promoter sequences can be from any organism, provided that they are functional in the host organism. Inducible promoters as used in the constructs of the present invention can use one or more portions or one or more domains of the aforementioned promoters or other inducible promoters fused to at least a portion of a different promoter that operates in the host organism to confer inducibility on a promoter that operates in the host species.

A variety of gene promoters that function in cyanobacteria can be utilized in expression vectors, including, but not limited to: 1) the lac, tac, and trc promoters that are inducible by the addition of isopropyl β-D-1-thiogalactopyranoside (IPTG), 2) promoters that are naturally associated with transposon- or bacterial chromosome-borne antibiotic resistance genes (neomycin phosphotransferase, chloramphenicol acetyltrasferase, spectinomycin adenyltransferase, etc.), 3) promoters of various heterologous bacterial and native cyanobacterial genes, 4) promoters from viruses and phages, and 5) synthetic promoters. Promoters isolated from cyanobacteria that have been used successfully include the following:

-   -   secA (secretion; controlled by the redox state of the cell)     -   rbc (Rubisco operon)     -   psaAB—(PS I reaction center proteins; light regulated)     -   psbA—(D1 protein of PSII; light-inducible)     -   nirA—(nitrate reductase, NH3/NO3 regulated)

Likewise, a wide variety of transcriptional terminators can be used for expression vector construction. Examples of possible terminators include, but are not limited to, psbA, psaAB, rbc, secA, and T7 coat protein.

Transformation vectors preferably also include a selectable marker, such as but not limited to a drug resistance gene, an herbicide resistance gene, a metabolic enzyme or factor required for survival of the host (for example, an auxotrophic marker), etc. Transformed cells can be optionally selected based upon the ability to grow in the presence of the antibiotic or other selectable marker under conditions in which cells lacking the resistance cassette or auxotrophic marker would not grow. In some embodiments a non-selectable marker may be present on a vector, such as a gene encoding a fluorescent protein or enzyme that generates a detectable reaction product. In an alternative transformation strategy, selectable or non-selectable markers can be provided on a separate construct, where both the gene-of-interest construct and the selectable marker construct are used together in transformation protocols, and selected transformants are analyzed for co-transformation of the construct that includes the gene-of-interest (see, for example, Kindle (1990) Proc. Natl. Acad. Sci. USA 87: 1228-32; Jakobiak, et al. (2004) Protist 155:381-93).

Methods for Producing Branched-Chain Alcohols

A further aspect of the invention is a method for producing a branched-chain alcohol in which the method includes culturing a recombinant photosynthetic microorganism as provided herein to produce a branched-chain alcohol. The photosynthetic microorganism can be, for example, a photosynthetic microorganism that carries a heterologous gene encoding at least one polypeptide that catalyzes a substrate to product conversion that leads to the synthesis of isobutanol, a heterologous gene encoding at least one polypeptide that catalyzes a substrate to product conversion that leads to the synthesis of 2-methyl-1-butanol, or a heterologous gene encoding at least one polypeptide that catalyzes a substrate to product conversion that leads to the synthesis of 3-methyl-1-butanol. In some preferred embodiments, a photosynthetic microorganism used for the production of one or more branched-chain alcohols includes a heterologous nucleic acid sequence encoding a branched-chain 2-ketoacid decarboxylase and a heterologous nucleic acid sequence encoding an alcohol dehydrogenase.

The photosynthetic microorganism can be cultured mixotropically, in which the microorganism is grown in the light for at least a portion of the growth period and is also supplied with a reduced carbon source, or can be cultured photoautrophically. In some embodiments the photosynthetic microorganism is cultured under photoautotrophic conditions, in which the culture lacks a reduced carbon source and the organism is supplied with or exposed to light for at least a portion of the time it is in culture. The photoautotrophic culture is in some embodiments provided with inorganic carbon such as CO₂, carbonic acid, or a carbonate salt. An inorganic carbon source such as flue gas or air can also be provided.

In preferred embodiments, the methods for producing a branched-chain alcohol include culturing a photosynthetic microorganism photoautotrophically using inorganic carbon as the sole source of carbon for incorporation into biomass or algal products. In these embodiments, the culture medium for photoautotrophic growth lacks sugars, organic acids, or other forms of reduced carbon that can be used as an energy source, although it may contain one or more reduced carbon molecules in amounts that are insufficient for supplying the culture with a source of energy for supporting cell division and/or biomass accumulation (for example, a vitamin such as thiamine).

A photosynthetic microorganism used in the methods that includes a heterologous nucleic acid sequence encoding a branched-chain 2-ketoacid decarboxylase and a heterologous nucleic acid sequence encoding an alcohol dehydrogenase may further include at least one heterologous nucleic acid sequence encoding one or more of an acetolactate synthase (EC 2.2.1.6), a ketol-acid reductoisomerase (EC 1.1.1.86), or dihydroxyacid dehydratase EC 4.2.1.9. The culture is in some preferred embodiments produces isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol, or a combination thereof.

In some embodiments, the photosynthetic microorganism produces 3-methyl-1-butanol. In some embodiments, the photosynthetic microorganism produces 3-methyl-1-butanol and is engineered to include at least one heterologous nucleic acid sequence encoding one or more of the enzymes 2-isopropylmalate synthase (EC 2.3.3.13), 3-isopropylmalate dehydratase (EC 4.2.1.33), or 3-isopropylmalate dehydrogenase (EC 1.1.1.85) in addition to a heterologous nucleic acid sequence encoding a branched-chain 2-ketoacid decarboxylase and a heterologous nucleic acid sequence encoding an alcohol dehydrogenase.

In some embodiments, the photosynthetic microorganism produces 2-methyl-1-butanol. In some embodiments, the photosynthetic microorganism produces 2-methyl-1-butanol and is engineered to include at least one nucleic acid sequence encoding one or more of the enzymes homoserine dehydrogenase (EC 1.1.1.3), homoserine kinase (EC 2.7.1.39), threonine synthase (EC 4.2.3.1), or threonine ammonia-lyase (EC 4.3.1.19) in addition to a heterologous nucleic acid sequence encoding a branched-chain 2-ketoacid decarboxylase and a heterologous nucleic acid sequence encoding an alcohol dehydrogenase.

In some embodiments, the method includes recovering the branched-chain alcohol from the culture medium, for example, using methods such as liquid-liquid extraction, gas stripping, steam stripping, or pervaporation (Qureshi, et al., Biotechnol Prog. 15:594-602 (1999), Jitesh, et al., Bioseparation 9:145-154 (2000), Ezeji, et al., Bioprocess Biosyst Eng. 27: 207-214 (2005), Qureshi, et al., Bioprocess Biosyst Eng. 27: 215-222 (2005), Ezeji, et al., J Ind Microbiol Biotechnol 34: 771-777 (2007), Izak, et al., Appl Microbiol Biotechnol. 78: 597-602 (2008), Zeng, et al., J Ind Microbiol Biotechnol 36: 1127-1138 (2009). Any of these methods may be used in combination with distillation. The methods can in some embodiments be used for extraction of products from a continuous culture.

In a further aspect, included within the scope of the invention is a branched-chain alcohol made by the methods provided herein. The branched-chain alcohol produced by a recombinant photosynthetic microorganism can be, for example, isobutanol, 2-methyl-1-butanol, or 3-methyl-1-butanol. Also included are compositions that include a branched-chain alcohol produced by a recombinant photosynthetic organism as disclosed herein. The composition can be, for example, a fuel or solvent.

In another embodiment of this invention, the branched-chain alcohols can be chemically dehydrated to the corresponding alpha-olefins. For example, isobutanol can be used to produce 2-methylpropene (isobutylene) or isooctane, 2-MBO can be used to produce 2-methyl-1-butene, and 3-MBO can be used to produce 3-methyl-1-butene. Such compounds have uses that are known in the art, for example, in the petroleum industry. Such compounds can be further used to produce other compounds, for example, both 2-methyl-1-butene and 3-methyl-1-butene can be used to produce 3,3,5-trimethylpentane. All of the branched-chain alcohols can also be used to produce their corresponding ethers. The esters from certain compounds, such as 2-MBO or 3-MBO, can be used as flavors or fragrances.

The following examples are offered to illustrate but not to limit the invention.

Example 1 Production of Isobutanol, 2-Methyl-1-Butanol, and 3-Methyl-1-Butanol in the cyanobacterium Synechococcus

A DNA fragment comprising a functional operon was synthesized such that it contained the following elements in the given order: 1) the trc promoter, the Saccharomyces cerevisiae pyruvate decarboxylase gene (PDC1, GenBank Accession No. X77316) codon-optimized for expression in Synechococcus elongatus PCC 7942, the S. elongatus KaiBC intergenic region, the S. cerevisiae alcohol dehydrogenase gene (ADH2, GenBank Accession No. J01314) also codon-optimized for expression in S. elongatus, and the rrnB terminator. The nucleotide sequence of this functional operon is provided in SEQ ID NO:1 (FIG. 3). Codon optimization was performed by the use of the “Gene Designer” (version 1.1.4.1) software program provided by DNA2.0, Inc. The plasmid pSGI-BL3 was constructed by inserting the operon between SpeI and SacI restriction site in the vector pAM2314 (Mackey, et al., Methods Mol. Biol. 362:115-29), which enables transformation of S. elongatus via integration into the “NS1” site of the S. elongatus PCC 7942 chromosome.

An additional vector was constructed to enable the expression and testing of a different 2-ketoacid decarboxylase gene, the Lactococcus lactis KDCa gene (GenBank Accession No. AY548760), in combination with the codon-modified S. cerevisiae ADH2 gene. The nucleotide sequence of this KDCa/ADH2 functional operon is provided in SEQ ID NO:2 (FIG. 4). This operon was placed between the SpeI and SacI restriction sites in the plasmid pAM2314 to form pSGI-BL19.

An additional vector was constructed to enable the expression and testing of a different alcohol dehydrogenase gene (the S. cerevisiae ADH6 gene with gene ID number 855368, encoding the protein provided as Genbank accession number NP_(—)014051.1 GI:6323980) in combination with the codon-optimized S. cerevisiae PDC1 gene. The nucleotide sequence of this PDC1/ADH6 functional operon is provided in SEQ ID NO:3 (FIG. 5). This operon was placed between the SpeI and SacI restriction sites in the plasmid pAM2314 to form pSGI-BL20.

An additional vector was constructed to enable the expression and testing of the L. lactis KDCa gene in combination with the S. cerevisiae ADH6 gene. The nucleotide sequence of this KDCa/ADH6 functional operon is provided in SEQ ID NO:4 (FIG. 6). This operon was placed between the SpeI and SacI restriction sites in the plasmid pAM2314 to form pSGI-BL21.

Synechococcus elongatus PCC 7942 cells were transformed with plasmids pSGI-BL3, pSGI-BL19, pSGI-BL20, and pSGI-BL21 as described by Golden and Sherman (J. Bacteriol. 158:36-42). Both recombinant and wild-type control strains were pre-cultivated in 20 mL of BG-11 medium to mid-log phase (OD730 nm=0.7-0.9) on a rotary shaker (150 rpm) at 30° C. with constant illumination (30 μEinsteins m-2 sec-1).

BG-11 medium was made by combining in a total volume of one liter: 10 ml of ‘100×BG-11’; 1 ml of 6 mg/ml Ferric ammonium citrate; 1 ml of 2% Na₂CO₃: and 1 ml of 3.05% K₂HPO₄. The components of ‘100×BG-11’ were 149.60 g of NaNO₃, 7.49 g MgSO₄.7H₂O, 3.60 g CaCl₂.2H₂O, 0.60 g citric acid (or 0.89 g Na-citrate, dehydrate); 1.12 ml 0.25M Na₂EDTA, pH 8.0; and 100 ml Trace minerals in a final volume of one liter. (Trace minerals solution included: 2.86 g/L H₃BO₃, 1.81 g/L MnCl₂.4H₂O; 0.222 g/L ZnSO₄.7 H₂O; 0.39 g/L Na₂MoO₄.2H₂O; 0.079 g/L CuSO₄.5H₂O; and 0.0494 g/L Co(NO₃)₂.6H₂O per liter.)

Mid-log phase cultures were inoculated in BG-11 containing 1 mM IPTG to obtain 40 mL of culture having an initial culture OD730 nm of 0.3-0.4). Cultivation was performed under the same conditions as pre-cultivation. Spectinomycin (5 μg/ml) was included in recombinant cultures as appropriate. Four mL of culture were collected every 48 hours and centrifuged at 6,000 g for 10 min. Culture supernatants were transferred into clean 1.5 mL microfuge tubes for gas chromatographic analysis.

2-methyl-1-butanol and 3-methyl-1-butanol were separated from the culture supernatant by liquid-liquid extraction, using 1 volume of culture supernatant to 2 volumes of CH₂Cl₂, for gas chromatography-mass spectrometry analysis. A 1 uL sample was injected at a 20:1 split ratio onto an Rtx-624 column (Restek, 20 m×180 μm×1 μm), which was equilibrated for 0.5 min and then operated using the following temperature gradient: 70° C. for 1 min, 10° C./min to 110° C. for 0.5 min and then 20° C./min to 140° C. for 0.5 min, 7.5 min run time at 140° C., and 2 min post run time at 200° C. (0.75 mL/min He).

For isobutanol analysis, the culture supernatant was passed through 0.2 μm PVDF filter and then analyzed directly by gas chromatography using flame ionization detection. An HP-Innowax column (Agilent, 15 m×250 μm×0.25 μm) was equilibrated for 0.5 min and then operated using the following temperature gradient: 35° C. for 2 min, 25° C./min to 180° C. for 0.2 min, 8 min run time and 2 min post run time at 220° C. (0.75 ml/min He). A 1 uL sample was injected at a 40:1 split ratio with a 250° C. injection port temperature.

Results indicating the levels of 2-methyl-1-butanol, 3-methyl-1-butanol, and isobutanol in Synechococcus elongatus PCC 7942 cultures 96 hours after culture inoculation and induction are shown in Table 1.

TABLE 1 Branched-chain alcohol production (in μM) in Synechococcus elongatus PCC 7942. Wild-type pSGI-BL3 pSGI-BL19 pSGI-BL20 pSGI-BL21 PCC 7942 (PDC1/ADH2) (KDCa/ADH2) (PDC1/ADH6) (KDCa/ADH6) 2-Methyl-1- ND ND ND ND 31 Butanol 3-Methyl-1- ND ND 6.5 ND 103 Butanol Isobutanol ND ND 18 ND 394 Note: ND represents “not detected” (<5 uM). 2-Methyl-1-Butanol and 3-Methyl-1-Butanol were identified and quantified by GC-MS. Isobutanol was identified and quantified by GC-FID.

Example 2 Production of Isobutanol, 2-Methyl-1-Butanol, and 3-Methyl-1-Butanol in the cyanobacterium Synechocystis

The functional operon (expression cassette) containing the codon-modified S. cerevisiae PDC1 and ADH2 genes as represented in SEQ ID NO:1 was digested by restriction enzymes Bgl II and SacI and inserted into plasmid pSGI-YC3 between the restriction sites BamHI and SacI to form plasmid pSGI-BL7, which enables integration of the functional operon into the Synechococcus sp. PCC 6803 chromosome at the “RS1” recombination site (Williams, Methods Enzymol. 167:766-778). Plasmid pSGI-BL22 contains the S. cerevisiae codon-modified PDC1 and native ADH6 genes as represented in SEQ ID NO:3 and was made by inserting a SpeI/SacI fragment from plasmid pSGI-BL20 into SpeI/SacI-digested pSGI-YC3. Plasmid pSGI-BL23 contains the L. lactis KDCa and S. cerevisiae native ADH6 genes as represented in SEQ ID NO:4 and was made by inserting a SpeI/SacI fragment from plasmid pSGI-BL21 into SpeI/SacI-digested pSGI-YC3. Plasmid pSGI-BL24 contains the L. lactis KDCa and codon-modified S. cerevisiae ADH2 genes as represented in SEQ ID NO:2 and was made by inserting a SpeI/SacI fragment from plasmid pSGI-BL19 into SpeI/SacI-digested pSGI-YC3.

Synechocystis PCC 6803 cells were transformed with plasmids pSGI-BL7, pSGI-BL22, and pSGI-BL23 as described by Zang, et al. (Microbiology 45:241-245). Both recombinant and wild-type control strains were pre-cultivated in 20 mL of BG-11 medium to mid-log phase (OD_(730 nm)=0.7-0.9) on a rotary shaker (150 rpm) at 30° C. with constant illumination (30 μEinsteins m⁻² sec⁻¹). Mid-log phase cultures were inoculated in BG-11 containing 1 mM IPTG to obtain 40 mL of culture having an initial culture OD_(730 nm) of 0.3-0.4). Cultivation was performed under the same conditions as pre-cultivation. Kanamycin (5 μg/ml) was included in recombinant cultures as appropriate. Four mL of culture were collected every 48 hours and centrifuged at 6,000 g for 10 min. Culture supernatants were transferred into clean 1.5 mL microfuge tubes for gas chromatographic analysis as described in Example 1.

Results indicating the levels of 2-methyl-1-butanol, 3-methyl-1-butanol, and isobutanol present in Synechocystis PCC 6803 cultures 144 hours after culture inoculation are shown in Table 2.

TABLE 2 Branched-chain alcohol production (in μM) in Synechocystis sp. PCC 6803. Wild-type pSGI-BL7 pSGI-BL24 pSGI-BL22 pSGI-BL23 PCC 6803 (PDC1/ADH2) (KDCa/ADH2) (PDC1/ADH6) (KDCa/ADH6) 2-Methyl-1- ND ND ND ND 28 Butanol 3-Methyl-1- ND ND ND ND 43 Butanol Isobutanol ND ND 10.1 ND 188 Note: ND indicates “not detected” (<5 uM). 2-Methyl-1-Butanol and 3-Methyl-1-Butanol were identified and quantified by GC-MS. Isobutanol was identified and quantified by GC-FID.

Example 3 Enhanced Production of Branched-Chain Alcohols in Strains of Synechocystis sp. by Overexpression of an Acetolactate Synthase Gene

A 1.6-kbp DNA fragment comprising the coding region of the acetolactate synthase gene from Synechocystis sp. PCC 6803 (ilvB, Cyanobase gene designation s111981) was amplified from genomic DNA using PCR with primers ilvB-5 (GTTGCACATGTTAGGGCA AATGAACACCGCAGACC SEQ ID NO:5) and ilvB-3 (CTACGTTAACGACAGAGATCT TTATTCCCAAATTTCACAGGCCA; SEQ ID NO:6). This PCR fragment was digested with the restriction enzyme PciI and BglII and the ilvB gene coding region was then inserted into the expression cassette of pSGI-BL27 between the NcoI site and BglII site to yield pSGI-BL34. The expression cassette comprising the trc promoter, the ilvB coding sequence and the rps14 terminator is provided as SEQ ID NO:7.

The pSGI-BL34 vector was transformed into wild-type Synechocystis sp. PCC 6803 to form strain SGC-BL34-1 and into Synechocystis sp. strain pSGI-BL23-1 (see Example 2) to form strain SGC-BL23-34-1 according to Zang et al., J. Microbiology (2007) 45:241-245. Insertion of the ilvB gene expression cassette into the “RS2” recombination site (Aoki, et al., J. Bacteriol (1995) 177:5606-5611) through homologous recombination was confirmed by PCR screening of insert and insertion site. The strains were then grown in liquid BG-11 medium and tested for the production of branched-chain alcohols. All liquid medium growth conditions used a rotary shaker (150 rpm) at 30° C. with constant illumination (60 μE·m-2·sec-1). Cultures were inoculated in 25 mL of BG-11 medium containing spectinomycin (10 μg/mL) and/or kanamycin (5 μg/mL) accordingly and grown to a sufficient density (minimal OD730 nm=1.6-2.0). Cultures were then used to inoculate 100 mL BG-11 medium in 250 mL polycarbonate flasks to OD730 nm=0.4-0.5 and incubated overnight. 45 mL of overnight culture at OD730 nm=0.5-0.6 were added to new 250-mL flasks, some of which were induced with 1 mM IPTG. 2 mL samples were taken at 0, 48, 96 and 144 hours post induction and processed as described in Example 2. GC results indicating secreted levels of branched-chain alcohols after 144 hours are shown in Table 3.

TABLE 3 Branched-Chain Alcohol production (μM) in strains derived from Synechocystis sp. PCC 6803. Strain Parent Added Plasmid Transgenes 2-MBO 3-MBO i-BuOH SGC- PCC pSGI-BL34 ilvB ND ND ND BL34-1 6803 SGC- PCC pSGI-BL34 KDCa + 77.6 215.8 1040.5 BL23-1 6803 ADH6 SGC- SGC- pSGI-BL34 ilvB + KDCa + 97.4 250.7 1137.1 BL23-34-1 BL23-1 ADH6 2-MBO refers to 2-methyl-1-butanol 3-MBO refers to 3-methyl-1-butanol i-BuOH refers to isobutanol 

The invention claimed is:
 1. A recombinant photosynthetic microorganism comprising at least one heterologous nucleic acid sequence that encodes a branched-chain 2-ketoacid decarboxylase, wherein the recombinant photosynthetic microorganism produces a branched-chain alcohol.
 2. The recombinant photosynthetic microorganism of claim 1, wherein the branched-chain 2-ketoacid decarboxylase is selected from the group consisting of a Saccharomyces cerevisae PDC1 gene product, a Pichia stipitis PDC1 gene product, a Saccharomyces cerevisae PDC5 gene product, a Saccharomyces cerevisae PDC6 gene product, a Saccharomyces cerevisae ARO10 gene product, a Saccharomyces cerevisae THI3 gene product, a Pichia stipitis PDC 3-6 Kivd gene product, a Pichia stipitis PDC2 gene product, a Mycobacterium tuberculosis KDC gene product, a Lactococcus lactis KDCa gene product, and a variant or homolog of any thereof.
 3. The recombinant photosynthetic microorganism of claim 2, wherein the heterologous nucleic acid sequence encodes Lactococcus lactis KDCa gene product or a variant thereof.
 4. The recombinant photosynthetic microorganism of claim 2, wherein the heterologous nucleic acid sequence encodes a Lactococcus lactis KDCa-S286Y, KDCa S286U, or F381W gene product, or a variant of any thereof.
 5. The recombinant photosynthetic microorganism of claim 2, wherein the heterologous nucleic acid sequence encodes a Pichia stipitis PDC3-6 gene product or a variant thereof.
 6. The recombinant photosynthetic microorganism of claim 1, wherein the heterologous nucleic acid sequence further encodes an alcohol dehydrogenase.
 7. The recombinant photosynthetic microorganism of claim 6, wherein the heterologous nucleic acid sequence encodes a branched-chain alcohol dehydrogenase.
 8. The recombinant photosynthetic microorganism of claim 6, wherein the alcohol dehydrogenase is selected from the group consisting of a Saccharomyces cerevisae ADH1 gene product, a Saccharomyces cerevisae ADH2 gene product, a Saccharomyces cerevisae ADH3 gene product, a Saccharomyces cerevisae ADH6 gene product, a Saccharomyces cerevisae ADH7 gene product, a Saccharomyces cerevisae GRE2 gene product, a Saccharomyces cerevisae SFA1 gene product, a Saccharomyces cerevisae YPR1 gene product, a Pichia stipitis AdH3 gene product, a Pichia stipitis ADH6 gene product, a Pichia stipitis ADH7 gene product, a GRE2 gene product, a Mycobacterium tuberculosis ADH1 gene product, a Mycobacterium tuberculosis ADH gene product, a Mycobacterium tuberculosis ADHb gene product, an E. coli yqhD gene product, an Equus caballus ADHE gene product, and a variant or homolog of any thereof.
 9. The recombinant photosynthetic microorganism of claim 8, wherein the heterologous nucleic acid sequence encodes a Saccharomyces cerevisae ADH6 gene product.
 10. The recombinant photosynthetic microorganism of claim 2, wherein said at least one heterologous nucleic acid sequence further encodes one or more gene products selected from the group consisting of an acetolactate syntase gene product, a ketol-acid reductoisomerase gene product, and a dihydroxyacid dehydratase gene product, and further wherein said recombinant photosynthetic microorganism produces isobutanol, 2-methyl-1-butanol, or 3-methyl-1 butanol.
 11. The recombinant photosynthetic microorganism of claim 10, wherein said one or more gene products is an acetolactate synthase gene product.
 12. A method for producing a branched-chain alcohol, said method comprising: (i) providing a culture medium comprising a carbon source, (ii) culturing the recombinant photosynthetic microorganism of claim 1 in said culture medium under photoautotrophic conditions, and (iii) recovering a branched-chain alcohol from said culture medium.
 13. The method of claim 12, wherein the culture medium provides inorganic carbon as substantially the sole carbon source.
 14. The method of claim 13, wherein the inorganic carbon in the culture medium is CO₂.
 15. The method of claim 12, wherein the branched-chain alcohol is selected from the group consisting of isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol.
 16. The method of claim 15, wherein said at least one heterologous nucleic acid sequence further encodes one or more gene products selected from the group consisting of an acetolactate synthase gene product, a ketol-acid reductoisomerase gene product, and a dihydroxyacid dehydratase gene product.
 17. The method of claim 15, wherein said wherein said at least one heterologous nucleic acid sequence further encodes an acetolactate synthase gene product.
 18. The method of claim 15, wherein the branched-chain alcohol is isobutanol.
 19. The method of claim 15, wherein the branched-chain alcohol is 2-methyl-1-butanol.
 20. The method of claim 19, wherein said at least one heterologous nucleic acid sequence further encodes one or more gene products selected from the group consisting of a homoserine dehydrogenase gene product, a homoserine kinase gene product, a threonine synthase gene product, and a threonine ammonia-lyase gene product.
 21. The recombinant photosynthetic microorganism of claim 1, wherein the photosynthetic microorganism is a eukaryotic microalga.
 22. The recombinant photosynthetic microorganism of claim 21, wherein the photosynthetic microorganism is selected from the group consisting of Achnanthes, Amphiprora, Amphora, Ankistrodesmus, Asteromonas, Boekelovia, Borodinella, Botryococcus, Bracteococcus, Chaetoceros, Carteria, Chlamydomonas, Chlorococcum, Chlorogonium, Chlorella, Chroormonas, Chrysosphaera, Cricosphaera, Crypthecodinium, Cryptomonas, Cyclotella, Dunaliella, Ellipsoidon, Emiliania, Eremosphaera, Ernodesmius, Euglena, Franceia, Fragilaria, Gloeothamnion, Haematococcus, Halocafeteria, Hymenomonas, Isochrysis, Lepocinclis, Micractinium, Monoraphidium, Nannochloris, Nannochloropsis, Navicula, Neochloris, Nephrochloris, Nephroselmis, Nitzschia, Ochromonas, Oedogonium, Oocystis, Ostreococcus, Pavlova, Parachlorella, Pascheria, Phaeodactylum, Phagus, Platymonas, Pleurochrysis, Pleurococcus, Prototheca, Pseudochlorella, Pyramimonas, Pyrobotrys, Scenedesmus, Skeletonema, Spyrogyra, Stichococcus, Tetraselmis, Thalassiosira, Viridiella, and Volvox.
 23. The recombinant photosynthetic microorganism of claim 1, wherein the photosynthetic microorganism is a cyanobacterium.
 24. The recombinant photosynthetic microorganism of claim 23, wherein the photosynthetic microorganism is selected from the group consisting of Chamaesiphon, Chroococcus, Cyanobacterium, Cyanobium, Dactylococcopsis, Gloeobacter, Gloeocapsa, Gloeothece, Microcystis, Prochlorococcus, Prochloron, Synechococcus, Synechocystis, Chroococcidiopsis, Cyanocystis, Dermocarpella, Myxosarcina, Pleurocapsis, Stanieria, Xenococcus, Arthrospira, Borzia, Crinalium, Geitlerinema, Halospirulina, Leptolyngbya, Limnothrix, Lyngbya, Microcoleus, Oscillatoria, Planktothrix, Prochlorothrix, Pseudanabaena, Spirulina, Starria, Symploca, Trichodesmium, and Tychonema, Anabaena, Anabaenopsis, Aphanizomenon, Calothrix, Cyanospira, Cylindrospermopsis, Cylindrospermum, Nodularia, Nostoc, Chlorogloeopsis, Fischerella, Geitleria, Nostochopsis, Iyengariella, Stigonema, Rivularia, Scytonema, and Tolypothri.
 25. The recombinant photosynthetic microorganism of claim 24, wherein the photosynthetic microorganism is Synechococcus or Synechocystis.
 26. The method of claim 12, wherein the photosynthetic microorganism is a eukaryotic microalga.
 27. The method of claim 26, wherein the photosynthetic microorganism is selected from the group consisting of Achnanthes, Amphiprora, Amphora, Ankistrodesmus, Asteromonas, Boekelovia, Borodinella, Botryococcus, Bracteococcus, Chaetoceros, Carteria, Chlamydomonas, Chlorococcum, Chlorogonium, Chlorella, Chroomonas, Chrysosphaera, Cricosphaera, Crypthecodinium, Cryptomonas, Cyclotella, Dunaliella, Ellipsoidon, Emiliania, Eremosphaera, Ernodesmius, Euglena, Franceia, Fragilaria, Gloeothamnion, Haematococcus, Halocafeteria, Hymenomonas, Isochrysis, Lepocinclis, Micractinium, Monoraphidium, Nannochloris, Nannochloropsis, Navicula, Neochloris, Nephrochloris, Nephroselmis, Nitzschia, Ochromonas, Oedogonium, Oocystis, Ostreococcus, Pavlova, Parachlorella, Pascheria, Phaeodactylum, Phagus, Platymonas, Pleurochrysis, Pleurococcus, Prototheca, Pseudochlorella, Pyramimonas, Pyrobotrys, Scenedesmus, Skeletonema, Spyrogyra, Stichococcus, Tetraselmis, Thalassiosira, Viridiella, and Volvox.
 28. The method of claim 12, wherein the photosynthetic microorganism is a cyanobacterium.
 29. The method of claim 28, wherein the photosynthetic microorganism is selected from the group consisting of Chamaesiphon, Chroococcus, Cyanobacterium, Cyanobium, Dactylococcopsis, Gloeobacter, Gloeocapsa, Gloeothece, Microcystis, Prochlorococcus, Prochloron, Synechococcus, Synechocystis, Chroococcidiopsis, Cyanocystis, Dermocarpella, Myxosarcina, Pleurocapsis, Stanleria, Xenococcus, Arthrospira, Borzia, Crinalium, Geitlerinema, Halospirulina, Leptolyngbya, Limnothrix, Lyngbya, Microcoleus, Oscillatoria, Planktothrix, Prochlorothrix, Pseudanabaena, Spirulina, Starria, Symploca, Trichodesmium, and Tychonema, Anabaena, Anabaenopsis, Aphanizomenon, Calothrix, Cyanospira, Cylindrospermopsis, Cylindrospermum, Nodularia, Nostoc, Chlorogloeopsis, Fischerella, Geitleria, Nostochopsis, Iyengariella, Stigonema, Rivularia, Scytonema, and Tolypothri.
 30. The method of claim 29, wherein the photosynthetic microorganism is Synechococcus or Synechocystis. 